A lot of Zenoah parts fit the RCMK's as I've put together some with their parts. The Zen coils are a big improvement over theirs. The bearings coming from the newer ones are junk so swap those out for some good ones. I'd ditch their pull-starter for the Zenoah Easy-Start.
Ordering from RCMK isn't bad as the shipments do arrive in a decent amount of time.
Bruce Lykens at Gizmo Motors doesn't distribute the RCMK line anymore but has some interchangeable parts. I have some pieces but not many.