Belt drives
I've had a belt drive on my Manx since 1994. (They're encouraged in European classic racing since unlike chains there's no source of oil leaks.)
I've broken one belt (stone damage) and replaced one. The current one is about five years old. Since there is no sign of fraying, I've left it alone. A spare belt takes no space anyway, coils down by "a deft double flick of the wrist" - familiar to cyclists coiling down tubular tyres - to a 4" toroid.
I run mine slacker than recommended, about 1" total up and down play. I haven't touched the adjuster in five years. Some guys dispense with adjusters (saving the odd ounce) and drill a single hole in the engine plates for the top bolt.
It evidently has a certain amount of shock absorption, since what eventually happens, with an ally engine pulley on straight splines in a transmission with no other shock absorption, is that the pulley grows slack on the mainshaft splines. I've only replaced one pulley.
Mine is run open (stones come in - stones go out again...) but they are also used fully enclosed on Commandos - stones don't come in.
For the record, last time on the dyno the Norton was putting out 52 bhp at the back wheel, with a virtually flat torque curve, 42 lb.ft, from 5000 to 7200.
Whether they would perform as well on a Vin twin, at much lower rpm, is moot. But the Vin also has an ESA, so maybe everything would balance out. What is certain is that if the belt drive on your Vin let go, it wouldn't wreck the castings, and a spare could easily be carried in the pocket of a Barbour jacket.