Ok I can only tell you what I know.
the bigger barrel/head and the ported ones that a few people do (with the bigger cooling fins) fit easier on the cy as the zenoah has to have the crankcase ground or filed down so they fit.

The 29cc isnt either good for low down power but poor at the top or its all at the top and wont idle.
Thats the reason why a few people have stopped selling the 29cc conversion (the chamber bellow the piston is too small) The 26 to 27 crank conversion seems to be a bit of a waste of ££££s too.