If it is a stock 40 hp I would suggest 6 part swaps to make it a 50hp.
and 2 more after that to make it equivalent to 45ss = 62 hp.
1. 50/60 hp carbs (1.5" dia throats)
2. 50/60 hp reed cages(stop height was different in some models 50 vs 40).
3. 50/60 hp cyl head smaller dia chamber with less cc's.
3a. mill 50 head for .515 chaber depth = 45ss spec.
note: milling 40hp head IS NOT the same.
4. set timing to 21degs(you can run more but 21 is a good place to start)
5. make a cowbell tuner
6. make sure power pack does not have rev limiter.
1-5 = about 62 hp @ about 6,200 rpms on pump gas
1987 is a good block and hp won't fall off significantly until 7,200 rpms that will put you in
low 50's with stock OMC original 21 p (made for 50twin has extra cup from factory to run engine a litte higher on the transom)
after that see Ron for a prop and he will get you dialed in
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