Dennis - I guess being a pragmatist, I'd start with a few questions... like:
1) where is the C/G with the K&B now? 2) Are you running the boat anchor K&B muffler or a chip or?? 3) is the tank compartment sealed or can you change tanks easily?
Personally, if it balances with the K&B and you have a metal uniflow, I'd leave it be and run the wet 2/2 run. I have a few and they ran great that way with an 11/5 wood, that if you measured, like all Rev-ups of that vintage, were a lot closer to 4 pitch than 5.
If you went to the OS 4stroke, I'd want tank access and room for a clunk... and hope that you were correcting for a rear CG while you were at it.
The FP40 can be fussy, but either that or the LA46 would be a a no brainer if the CG needs to go the other way, you save over an ounce from the K&B, and can swing the same if not larger props on the same fuel.
Well, that's my .02 for what it's worth.