My concern would be this, sounds like the aftermarket piston has a thin circlip,
know from experience how fat and nasty the rotax clips are so you put a fat clip
into a thin groove its going to be sticking up far enough it may not be locked in and pop out real easy.
I personally hate the stock clips, so hard to put in that you can end up bending them, not springy enough, they seem to bend first.
Good Luck