Depends on the engine. If your piston is shot, your con-rod more than likely is stretched/worn as well, so you would need to figure that into your cost.
If the engine was $150 or less when new, I'd just buy a new one and keep this one for parts.
If it's mechanically solid (no gouges in the sleeve/piston, conrod isn't sloppy), then maybe you can get the sleeve pinched to get some life out of it. $25 at rayaracing.com and he does good work. For a pull-start, ask for a mild pinch.