What're you thinking for belt with the overrange? Treats says stock dayco 7188 is 596mm long. I found this one for honda aero 50 that's like 634mm, and there's some other ones in a similar range. Pedal derbi motor belt is like 628, that'd probably be a good start too.
It'd just be finding that balance of taking advantage of bigger rear while still getting the variator to close fully. I was looking around and on the motomatic bikes it looks like they're rocking the scooter vario that fits, but I also saw a few pics where it looks like they are running the big malossi variotop from french bikes. I haven't had my hands on much french stuff, but i think ya'd needa get that spline in there for derbi crank, maybe cut that out from old fixed cheek and weld it in to the french one to make it fit? Idk