Maybe the latest RX21 models then I think.
I still have in my "old engine corner" both a crank case for the RX21 (Novarossi Speed I think it was) and the SX21 EVO5, and I can asure you they are not the same....
Porting in both crank cases are totally different...
The SX21 EVO5 is exactly the same as the MAX MF 21...
Porting is just a small issue and is no problem to fit another p/s set, timing makes it all. I know with all sets the intake has the same size and there are only differences in exhaust heights, even when using the same model from different series.
If you want a cheap rebuild of an old engine you can even fit a P/S set from a N21-3T of even from a GRP onroad engine (only the piston pin makes it less easy)