Oh it's not a belt drive at all - it's the C-V "Constant Velocity" principle that makes it similar to a snowmobile trans...Now, I am anything but an expert in these manners, being entirely self-trained over 15yrs and not even knowing proper names for half the parts, but the basics of how it works, much as I can figure out, is this.
You have two centrifugal clutches in there, one front, one rear....ok ?
your RPM's dont change all that much (if yer like me and smack the throttle to the stops and leave it there..) till one side or the other maxes out.
As the centri-force increases, the diameter<?> of the forward clutch/gear/thingie gets bigger and bigger while the back one gets smaller and smaller...and it works like adding teeth to a front sprocket while removing them from the back one, sort of....
That's a pretty damn bad job of trying to explain it, but it's hard to get across, just how it works, but it does work in much the way a snowmobile transmission does.
Funny thing about that, is that it goes pretty damn fast, all in all, once you have it fully wound out on level pavement, faster than a Tomos Bullet, at least, though I could not give it to you in MPH cause the speedo on mine was shot.
And of course...I still have two of them that'd go way cheap if anyone would care to haul em off, and one did run, last time I messed with it, plus a couple of spare bits and pieces including a coupla sets of new-in-package brake shoes.
Given the time, incentive and inclination, and perhaps a couple of better tools (though I can make do with what I have) I can make that Fa-50 of yours run Fred, if you wish to bring it up here for a weekend or so.
S'not a bad deal there, at least one 'ped guaranteed to run and two parts bikes for it, for dirt cheap, so give it some thought if ya will..