I have a 74 Velosolex in a box. I started restoration about 10yrs ago and it got side lined. I did restore the engine though, using European parts. I also have a Hongdu clone which also got upgraded with Euro parts. Its mounted
on a 24” Macargi cruiser. Has a Velo aura to it. I have run it quite a bit. They are great engines, very torquey, around 50cc.
As I remember 4K rpm is about max. Has a larger roller. Tire wear might be an issue with the grit type smaller rollers
turning at high speeds. Bike tires are not generally a hard compound. Some wear quickly. I have found the common
cruiser tire to do well. My favorite is the Maxis Hookworm. Wears like iron, has a big cross section that beds the roller
nicely, and a grippy cross hatch like tread. I dont remember the life of the 2.125 cruiser tire but it seems close to 2K
on my oak rollers, which saw a lots of dirt roads as well as asphalt. Not trying to be negative in any way about your
rollers. I applaud your innovations in the development of your FDs. Tire quality Im sure has improved in recent years.
The quest after the roller perfection, will be finding the best tire wear wise for it.