Thanks everyone, you are all a wealth of knowledge.
I think at this point it makes sense to pull the drive and at least have it professionally assessed to see if it's rebuildable vs a basket case. Considering the apparently less than common ratio it may be hit and miss to find something used.
As for finding and going with a 1.81,
36RPM@4000...Hmmm, what kind of difference are we talking? It has 15x17 props on it. The boat spends 98% of it's life at slow cruise for day trip getaways to the local sandbar, etc... only getting up on plane when we have a long stretch of lake to cover and we want to do it quickly on some longer trips.