Setting it up to TDC, if its a OHC (dobt it is) then you need to keep the cam it the correct possion, then take off the head, then remove the barrel, take out the gudion pin locking ring, then remove the gudion pin from the piston, fit new piston and rings (the correct way) fit new bigger gudion pin, fit new barrel without damaging the piston rings, re-fit head and cams etc.
Turn engine over by hand to make sure your not hitting the head with the piston. if all is ok then fire it up.