I have a webra 120 two stroke that runs but is pretty much worn out. I was thinking of rebuilding it. Any ideas where to begin and what parts are critical to replace? Also, where would parts be available for this engine.

Mecoa has some parts including cylinder liners and connecting rods. I don't see any pistons but there are individuals out there who are cutting new piston rings to order. Boca Bearing can probably set you up with the crankshaft bearings. Check your current cylinder liner. Is there a ridge at the top that you can hang your fingernail on? If so then you probably need a new liner. If not and the bearings aren't crunchy you my get some more use out of the engine with just a fresh piston ring. If you are wanting to make it "like new" you may have to shop around for an NOS piston.

Ring supplier: https://rmjmachineworx.com/

Use them for all my rebuilds.