posted 01-04-2014 02:36 PM ET (US)
IT's a 70, not a 75.Twin OMC 70 HP motors were pretty common on the 19s and 21s back in the 1970s. This boat probably had one motor die and a good deal was had on a used 85.
It makes sense; same controls and rigging, same bolt pattern for mounting. It would have been an easy swap.
The 85 would need a different pitch propeller, but assuming you learned what RPM each motor put out the same HP, there is no reason it would not work fine.
It does look odd though, doesn't it?