when a deal like this is spotted you got to be quick, i bought a 20hp yamaha early 1980s it was mint not a mark on it i got it for £450 had it out today irish sea it never missed a beat there are good deals out there.
Looks similar to my old (25hp) cowling etc. Will it still be 2 cylinder? Not that I need the extra power! There will be issues with any engine stored that long (I know from my interest in classic cars). I can also say that parts for these older engines aren't cheap.