So far....
As it does start there must be sparks at the plugs for it to start, is it safe to assume there are still sparks when it doesn't?
I'd imagine that they are still present so we have one out of three for a bang.
Next to look at is kind of obvious, there is an air bleed into the tank. You can tell if this is blocked or even partially blocked as any air space in the tank goes negative pressure so there's a little suck as the cap is opened. This tends to let the bike run for a while then as the negative pressure increases the to match the pumps output pressure..... You get the idea here, no more fuel flow or significant reduction, no bangs.
Possibly a check of the injectors pressure which might be an issue, a bit less likely as if it starts then there was the fuel and it was pressurised. Lose either and again no bangs.
Belts would be an extremely good idea, a belt snapping on a 2V is messy and on a 4V head is going to be twice as bad. Think many of these
with a shed load of parts going down the
I've already got a quattro motored ducati so it's all yours Mr G