Ring compressors really only get in the way with these engines. Fingers and fingernails is all you need, and the fact that the bottom of the cylinder sleeve is tapered helps out a lot.
I start out with the middle two pistons at TDC, pinch the rings to get them in the bores, drop the jugs down about halfway, rotate the crankshaft to bring the other two pistons up, pinch those rings to get them in, and you're done!