Make sure the ring gap is facing away from the port.
Compress the ring with one hand while sliding the cylinder with the other. There is a little notch in the piston where the ring gap should be. make sure the ring settles in there.
Once you have it sorta on, you can spin the crank with your hand to get con-rod and piston where you need it. You might need to wiggle the cylinder around the piston, DO NOT PUSH FROM SIDE TO SIDE ON THE CONNECTING ROD. THEY BEND VERY EASILY,
Torque the cylinder bolts in three stages using a cross pattern. The same with the head.
If you pulled the reed cages or intake maniforld I would do a leakdown test.