Like the guys say, it depends on the engine. If it is a ball bearing engine, then you can usually just swap a new piston/sleeve in it and get a "new engine". *Most* sleeve piston sets that are still available from the manufacturer will be cheaper than a whole new engine.
If it's a plain bearing ("bushing" in the case like a Fox 35), then bushing wear *might be a problem*. Then you got to make sure the rod is ok. A lot depends on how well te engine was treated.
Do you have a specific engine in mind?