As someone who has a fleet of museum pieces, if you can possibly see it running (better still - on the boat) will be worth it's weight in gold.
The other thing is tatty case only means it's seen the elements. I have a '72 Johnson which is so battered the bottom half of the cowl is broken and held in place by a couple of bits of aluminium (although about to be fixed as I have found someone breaking one!
) I trried selling it - even at £50 nobody wanted it, but it starts & runs second or third pull, even on fuel that has been lying for a year!
I would ask - how confident are you at repairs? Let me put this into perspective:
I have had 2 engines properly die on me:
- the 60 on the rib (bought for £200 untested but seen idling on muffs, shame I forgot to check the spark advance as the piston collided with the exhaust port...
and a wee 4Hp aux, bought told it was running for about £50 and it ran reasonably well until i accidentally got it slightly upside down, trapped aater for mthe exhaust got into the wrong bit of the powerhead and it siezed.
The 60 I then spent around another £800 totally rebuilding. OK, I paid what I would for another unknown used engine overall, but now have a "new" engine of known history.
The 4 I abandoned as the cost of a rebore would totally eclipse that of a replacement, and having had 4 years use out of it I reckoned £12.50 / yr rental wasn't a bad price!
Admittedly that is no help with choosing between your engines, but other things that make 2- stokes run like cr@p are knackered HT leads and blocked carb jets. Both are fixable at relatively low cost. Also have a look at the props / skegs etc - signs of being repeatedly grounded / smacked into rocks etc might imply the previous owner wasn't as careful as they might have been......
Hopefully this has given you a view from the "museum" end of ribnet.