I've used more modern standard cruiser frames with no trouble- By themselves they weigh only a pound or two more than a vintage road frame- but the enlarged down tube is a hassle. I set them up with shock forks- harder to find thread now, but out there- and 700 c wheels with big gears- 36 or smaller on the back which reduces vibration at cruisoing speeds-
also recently got a "supermount" with large studs and thick rubber inserts for the 66 build, because I now notice the vibration more than I used to, when I almost always ride a 50cc. Havem't got it on yet though- probably have to split the chain and use another- and that's always the stickler of the build as far as I'm concerned.
So still on the Marcargi Panther frame from 2009- and the 66 with 34 sprock
and then the Huffy frame - not terribly recent but dependable with the 50 cc and 39t
and I built and sold a 66 huffy matching frame with another alloy 34t last summer.