This will put the cat amongst the pigeons so-to-speak. The Yamaha 3 cylinder 2-stroke is a great engine, but finding one in decent condition is no easy task. Yes they come on Ebay and Boats and Outboards - but most look like they have seen better days.
What could be an option is to pick up a decent Yamaha 40hp and upgrade it to 50hp. They use the same block (693cc), but the exhaust manifold and carbs are a 1/4'' smaller.
Incidentally - there's was a Yamaha 50hp (long shaft) at www.seaandshore.co.uk
- ask for Ewen.