For single ring pistons (and maybe this is an old idea), but, I just thought of this and it works a treat
Modify a cable tie, grind away a little on one side
Put the cable tie in position, make sure the ring is "on the peg"
Tighten up the cable tie, again, make sure the ring stays on the peg
With the piston at the bottom of stroke put the cylinder down on the studs and sit it square on the piston, and having the piston at bottom dead centre makes this easy
Give a tap on the top of the cylinder, and not much of a tap in needed, it should go down very easily, and presto, ring in and cable tie on its way being pushed to the bottom
Cable tie off the bottom of the piston, ready to be cut and pulled out, you will need to turn the crank a little to get the piston up a bit to get some room to get in to do the cut, but that will be obvious when you are at that point