I would be wary about buying them from a pawn shop, unless they allow you to take apart and inspect the piston and cylinder walls (doubtful). There's a good chance they were abused and run to lean, thus scoring the cylinder walls.

As with any other source for used engines - you take your chances...

P.S. - No, I haven't converted an Echo, but here are a few who have: http://www.rcuniverse.com/showthread...08&forumid=257]