Didn’t say it’s a weak engine, but if you go tripling or quadrupling the h.p. of any machine there will be issues that come up. And considering how piston port parts are pretty hard to find these days any failure is catastrophic.
As for mounting a puch cylinder on a derbi, it requires opening the 2mm or so. On my eurokit with a head gasket before I went to a o ring, it did not like to seal and would occasionally blow out by the stud hole. I bet if I used a derbi metra head it would have been better. Oh and one more thing, starting a 74cc derbi with no decomp sucks! I got my starter clutches so dialed and it was still a bear, you walk next to it and then when it starts to engage you have to step on the pedal and put all of your weight down.
TLDR; get a flat reed and bolt on a metrakit. Easier, more fun and cheaper.