I looked at this very scenario over 10 years ago and Yamaha 50hp parts were as rare as rocking horse poo, so now it's like looking for the Holy Grail. Can't answer regards 70hp, etc.
My understanding is that model 40V and 50G shared the same block, so work on that premise. Even if you have the parts you need to remove the engine block to access the exhaust manifold. On a 1991 engine (28 years old) you run the very real risk of sheared bolts, drilling them out, and retapping. Best you're aware as it's not for the faint hearted.