1984cr125 low compression

I bought my brother a 84 cr125 for his first bike. The motor was saposably rebuilt 1-2 years ago. My brother overfilled the clutch,trans oil resivor. He fouled a plug. I relized what he did, I drained the oil put a new plug in and it ran good. But the bike has been getting harder, and harder to start. I checked everything and relized it has really low compression. 70psi The honda dealer said it should be 130-160. I dont think there is a seal or gasket out. The bike has always been hard to start. Even before he overfilled it. He always had to poush start it. Im thinking it needs new rings,and possibly a piston. My question is HOW HARD IS THIS TO DO? Should I even atempt it. I have never rebuilt an engine before. The manual I have shows you step, by step how to do this. But Im still a little apprehensive on doing it. What do you think? I really cant afford a $400 repair bill. If I do decide to do it should I take the piston and the cylnder to the dealer to make sure I get the right parts, and to get the cylnder honed out? I think it is already 040 over. Also is it possible to have the head machined down a little to get a little more compression out of the engine? Any help would greatly be apreciated!!!! Thanks Dave.