Mike, the ad describes that as a V-twin, not an opposed twin, but in the pics it looks like an opposed twin as you said. There can be significant problems, in particular fitting it into the bodywork. In the picture it looks as if the air cooling cowl is missing from the engine, and it is described as "good running order", which usually means it will run if you can start it and put up with the smoke.
You need to know the crankshaft diameter and length, whether the engine is complete, etc. After that you have to anticipate re-ringing the engine at a minimum, and more likely than not having to cobble the whole mower to fit the wrong shape of engine onto it. This is a project for a hobbyist who enjoys the tinkering more than he needs to do the mowing.
