Here's a dodge I have used in the past:
Cut the top and bottom out of a beer can (your choice on brand) slice the can lengthwise and wrap around the piston rings just above the rings then wrap the whole thing with wire to hold the rings in compression in the bottom of the ring groves.
Install the piston from the bottom side. hold the piston up to the cylinder and bump it in with a hammer handle. Install one wrist pin circlip. Stick the wristpin into the other side of the piston. Position the rod at top dead center. Place cylinder base gasket (if used) on the cylinder. Stuff a clean rag into the crankcase opening. Push the wrist pin thru the small end of the rod until it hits the circlip on the other side (make sure piston is installed correctly), install last circlip.