Motorcycle Repair: No Power, yamaha rt 180, piston rings

Question
I have a 1991 yamaha rt 180.  I also have another rt 180 that is a parts bike.  My running bike started smoking so.  I pulled the jug off the parts bike and it had a .50 over piston in it.  I traded jugs and pistons and bought new .50 over rings.  The bike cranked up good and sounds good.  It just dosen't have enough power to even shift through all the gears.  I dont know what to do next.  The sparkplug is new and the air filter is new.  I set the air mixture screw a 2.5 turns like it used to be set.  I need help.
Thanks
Joey P

Answer
Hi Joey,

Was the cylinder bored & honed when the new piston and piston rings were installed? If not, that's likely the problem.

Perform a compression check and a leak-down test to know the condition of the engine (sealing properties of the combustion chamber/crankcase).

Respectfully,
Mark Shively