The only thing I hate more than cowboy garages is ones that are full of ?
Unfortunately I think the Green Flag idiot probably killed it continuing to crank.I suspect just a few were missing initially then the continued cranking causing probable internal damage and stripped off more teeth as the pistons met the resistance of valves. If a 2 year old belt you may be LUCKY with a warranty claim but would need proof. Can you try to claim anything from Green Flag for their incompetent "mechanic"
Half of me says to put a new belt on and try after checking for injector damage, but the other half says you need to see what the valves and pistons look like first. Looking inside the engine while built up is nearly impossible. I did have access to very thin flexible borescopes but no longer.