You have asked a question that requires a very broad and comprehensive answer. It is impossible to get into details in a reply. I will offer very general advice.
When re-powering a boat with a new engine, the tasks that must be performed will vary. If the new engine is a different brand than old--as is proposed here--more problems must be faced. Usually there is not much compatibility among engine brands in rigging gear and accessories. The following items will likely have to be replaced if a new engine from a different manufacturer is to be rigged on an old boat:
RE-POWER NEW GEAR NEEDED- engine electrical harnesses for remote controls and instrumentation
- engine remote panels for ignition switch, safety lanyard, warning lamps, aural alarm sounders, and so on
- engine basic instrumentation such as tachometer and trim gauge
- engine remote throttle and shift controls
- fittings for fuel hoses
- required fuel filters
- required measured maximum vacuum pressure in fuel delivery hose
- propeller fit onto propeller shaft
- propeller pitch and diameter
Because the amount of associated rigging components is rather large and expensive, I recommend you solicit quotations for any new engines which will include the cost of all of those items (listed separately), and, if you are planning to have the engine installed and rigged by the selling dealer, to have the labor costs for mounting and rigging and configuring all those items included in the quoted price as a separate line item.
Note that many engine manufacturers require that the engine be installed by a certified dealer, and the dealer must either perform all the rigging or certify that the customer has performed the rigging in complete compliance with the required specifications if the manufacturer's warranty is to come into effect. Without dealer certified installation, the warranty coverage will not be applied in most cases.
When re-powering the boat fuel system should be very carefully assessed to make sure there are no air leaks, no restrictions, and no hidden problems. Problems in the fuel system of older boats can cause significant damage to modern outboard engines, and neglect of relatively inexpensive refits to the fuel can later cause huge engine repair bills--not covered by warranty.
In the case of this 20-year-old boat, the fuel system must be carefully inspected and likely will need new hoses, filters, and fittings. If the boat transom has the BIA standard engine hole layout, and if the hole location is in the proper position, and if, upon inspection, the holes are found to be well sealed, and if the transom, upon inspection, is found to be sound, strong, and free from any internal wood rot, softness, or decay, then the new engine can likely be bolted onto the transom using the existing mounting holes. When the old engine is removed from the transom, a very careful inspection and close assessment of the transom should be performed. Any deficiency in the transom should be repaired before the new engine is installed.
When the new engine is mounted, the engine mounting height should be discussed in advance and an agreement reached between the dealer and customer about the exact engine mounting height. Many dealers will, in the absence of such discussion, mount a new engine in the lowest possible position; this will be unlikely to be the best mounting height.
Once the engine is installed, a new propeller may be necessary. This is particularly true in the case of the SUZUKI four-stroke-power-cycle engines because of the use of unusual gear reduction ratio and a very large propeller aperture. I would anticipate needing to buy a new propeller. When soliciting bids for a new engine, ask that a sea trial be included and stipulate that a propeller or multiple propeller tests will be conducted until a suitable propeller is found and demonstrated in a sea trial. Although buying the propeller from the dealer may be slightly more expensive, if the dealer provides several propellers for testing, the added cost will be worthwhile.
Regarding engine remote controls: when re-powering consideration should be given to electrical or electronic shift and throttle controls (EST controls) if they are available as an option. Modern engines are often sold with EST remote controls. These EST controls are generally an improvement over the old-style mechanical controls linked by flexible cables. Re-fitting with EST controls may not be possible after engine purchase. Also, some models mandate EST controls.
Re-use of existing mechanical throttle and shift control may be possible. Generally the direction of motion of the linking cables and the particular fittings on the linking cables will vary among engine brands. Older shift and throttle controls and their associated cables that are about 20-years-old may be due for replacement, particularly if the controls and the cables were bottom-tier models. At one time Mercury was selling three tiers of remote controls, and the bottom-tier products were not especially great. Considering the total cost of a re-power with a 300-HP engine, I would not skimp on the remote controls.
Regarding engine instrumentation: when re-powering consideration should be given to modern electronic instrumentation using digital data networks. Literally all modern engines support this type of instrumentation. Among some brands and models, electronic digital networked instrumentation is mandatory.
Check the engine manufacturer specification for the engine cranking battery. It is typical for modern engines to have very specific requirements for the cranking battery, for the type of battery, and for the ability of the battery to provide a minimum cranking current in Amperes. Newer engines with four-stroke-power-cycle design will typically have higher compression ratios than older low-compression two-stroke-power-cycle engines, and will be more difficult to crank over during starting. If the present engine cranking battery in the boat does not meet specification or if it is an older battery, a new cranking battery should be provided.
If moving the battery location farther away from the engine, be CERTAIN that you consult the manufacturer's rigging guide to determine the proper minimum battery cable gauge necessary to comply with their specifications. Many modern engines will have trouble starting with low battery voltage during cranking. Note that it is never a good idea to include connections or splices or any other discontinuity in engine cranking battery cables. Buy the OEM battery cables of the proper gauge and length as stipulated in the engine manufacturer's rigging guide. Note this topic is covered in SMALL BOAT ELECTRICAL, where topics related to boat electrical installations are located. See
Conductor Size for Power Distributionviewtopic.php?f=9&t=578Also see the REFERENCE article
Conductor Size for Power Distributionhttp://continuouswave.com/whaler/refere ... rSize.html