I live on a small lake which had a 40hp max limit until a few years ago and the cottage owners voted to increase it to 60hp so I'm pretty familiar with the HP class you asking about. I've own several Evenrudes's Mercurys and a Yamaha over the years and watched many go by my home and many adrift with engine problems. This is my experience, IMHO .
1.) The Yamaha midrange 2 cycles are the most maintenance free and best performing (fastest) of the outboards of the 1990's.
2) A yamaha 2 cycle is faster than 4 cycle. My evidence is that my neighbor has a 40 2cycle that runs faster than another boat on the lake that is the EXACT same boat with a brand new 60hp yamaha 4 cycle. Hard to believe but I witnessed the race. There was a prop difference.
3) Seems like the newer yamaha 4 stroke owners are not as happy with their motors. Several have had early life problems and had to go back to the shop. This just seemed to never happen with the 2 strokes. I'm a big yamaha fan so I'm a little biased and I hope the few problems I'm aware of are due to poor winterization or improper breakin.
Back to your decision.
It's hard to compare a 1971 2 stroke that may have lost some of it's zip to a new yamaha that is a 4 stroke and lower hp but I agree with the earlier reply and I'd think you'll be losing a few MPH's by trading up to a newer 40 four stroke.
