If the piston/sleeve are expensive to buy new, you can get them serviced for $20, then replace the con-rod. If the bearings are still ok and there isn't anything else wrong with the engine, that's usually a cheaper alternative.
But, if your carb is starting to stick or your seals are dried up, your bearings are leaking or loose, then figure out if it's worth it in parts to rebuild. Most $150 engines are throw aways. If your lucky, you will get 8-12 gallons through them, then toss them. If your not lucky, you will get 2-5 gallons through them, be angry, then throw it away in spite.