i dunno if you figured out the taper cutting thing or not yet but this is how my toolmaker buddy taught me to do it
#1- put the crankshaft in the lathe in a 4-jaw so you can true that fucker up to the .0001 in terms of runout, put a dial indicator in the actual tool holder of the lathe, like, clamp it in where you would put the cutter, measure the runout at the taper itself, because even the best crank ever won't be that perfecto. when i do them, i will use a sacrificial crank and just bash it into place with a hammer. never done a vespa but with puch and tomos i clamp to the opposite side bearing race.
#2- now you loosen up the cross slide and get the angle close. snug it down a lil' bit and run the dial indicator across the taper. at this point i use a tiny hammer or usually the wrnch from the tool holder to just tap it back and forth while running the cross slide in and out across the taper. obviously you're going to go back and forth and you want the indicator to stay at '0' +/- .0002 or so. Then you rotate the crank like 90 degrees a couple times and repeat. Usually the tapers have some surface roughness so you'll never get it perfect but you get a good feel pretty quick for getting that cross slide dialed in on the the fuckin nuts. once the crossslide is tracking the taper perfect, tighten it down, check again, and remove the crank
#3- put the rotor in the 4-jaw, true that fucker up on the inner surface- this way you keep the factory balance. Take the dial indicator out and put a boring bar in the tool holder. cut the taper to whatever size you need. I usually figure what the smallest diameter will be and use a pin-gauge to guess and check but if you have a DRO or some fancy shit you can probably just get it right on that way. the actual bore diameter is probably only +/- .005 because you can shim your variator out to whatever you want by putting a shim between the flywheel and vari.
this is how i've done it for putting different-taper stators on different cranks.