The bike wouldn't be as heavy as some think, the Norton engine is surprisingly light other than the darn barrel and crankshaft of which both can be attended to for VMX and to trim some weight you only need 2 gears in the box, fast and faster
Those belt kits save a lot of weight too but quite a number of them give headaches mostly belts won't hang together although for VMX probably ok and of course he is using aluminium mounting plates everywhere.
The same pretty much applies to the A65 BSA engine too and with their shorter stroke I'm surprised more don't go for them for VMX - the A65 barrel especially is painfully heavy.