Just seen the video and since it sounds like it is spinning over very fast and easily on the starter so I too will take a guess at a broken cambelt and hopefully not anything else broken on the cams or pinion end of the crank that would keep some valves open while the engine spins.
Always a bit of a risk saying there is nothing wrong with the bevel box or the compensator but in this case it won't have anything to do with the engine not starting. Very much hoping it is just the cambelt, that the breakage is just a bad luck special and not to do with a sticking valve or such-like and that a spare is nearby (the alignment dowels are 3/16" so reversed drill bits are fairly ideal replacements for dowels.)
Good luck!
