If I were you I would invest a few bucks and find out whats wrong with the engine you have. You said it was given to you so you're no into it much money yet. He blew it up really doesnt describe the problem real well. You might just need a piston or a cylinder. Do you have a compression tester? Start there. For the price of another engine, shipping, and other incidentals you might be able to fix what you have and be more confident then just buying a random engine and hoping for the best. Just my couple cents worth anyways, you can take it for what its worth. Where are you anyways, location helps to know. People here are spread all over North America and a few in Europe too that can pint you in the right direction for parts and what not.
-Rustman