The cause is obvious. Your oil pump gear shattered.
The question is why ???
I'd be checking the gears it's meshed on. Those gears are notorious for coming loose on their shafts. Stripping woodruff keys etc. That could be the noise you were hearing.
Those oil pump gears don't just shatter like that. The meshing has to be incorrect. Is it also possible the oil pump is dead and jammed up. This could happen if it was running dry.
You said the bikes been stripped down. Whoever did it might have got something wrong.
Your cam is sitting in an oil bath. If the pump is dead it won't be filling the rocker box but it won't be emptying it either. It's hard to tell.
Your best best is to find a replacement sleeve for your cylinder. New piston, rings, piston pin. Hopefully your conrod survived the trauma.
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Did some trips.
Rode some bikes.
Fix them for a living.
Can't say anymore.
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