I would expect that unique parts for the marinized Ford engine would have to come from the company that produced the marinized version or from specialists in Ford marinized engines like
U.S. Marine Engines; the basic engine parts are probably available from Ford or aftermarket auto engine part suppliers. A
search on GOOGLE using the argument "ford marine engine 5.8-liter 351" returned hundreds of results. Those leads will probably bear more fruit than can be found here, as to the best of my recollection, RAGE 18 engine rebuilding has not been extensively covered in the forum or the Old Forum--there is little to no discussion in the archives.
The jet drive parts would come from the drive manufacturer
American Turbine (who appears to still be in business) or other jet-drive boat service shops.
I would be very surprised if a Boston Whaler authorized dealer could help you find these parts. The RAGE 18 was only in production for about two years. It represents a rather rare model of Boston Whaler boat. But I would not suspect that the engine and drive were especially unique to the RAGE. Since Boston Whaler did not manufacture either the engine or the drive, I would expect them to be components available on the marine jet drive market back in c. 1996.
I'd like to see a picture or two of that RAGE 18 if you have any.