tiger, with a bit of patience and perseverence you can compress the rings with your figers(top one first, then bottom one after you've pushed the top one through) and get the piston assembly in the bore. It helps to have the crank turned to a position where it allows the piston skirt to rest on top of the crankcase.
Alternately you can remove the gudgeon pin to remove the piston from the rod and do it that way.
Either way works, i've never used a ring compressor coz i never had one that small. Just make sure you size up your rings in the bore prior to assembly to make sure you have enough end gap; I've had a situation in the past where the rings were too tight in the bore(no end gap)and had to file the ends down a touch.
