First step in diagnosing possibly bent valves is to check the valve clearances. If they're a bit big, they're bent.
However, if valves have hit pistons, even if lucky not to bend them, they may have compressed the piston top and jammed the rings. Only real way to tell is to remove the pistons, which is a big job. Might be possible with the sump and head off.
Slow engine speed usually results in less damage, so cross fingers, fit a new belt and give it a try. Belt failure at slow speed is quite common as stresses tend to be higher then, at faster speeds some momentum reduces the belt stresses. When belts were a new thing, it used to be quite common for failure to occur as the engine was turned off, the little kick as it stops was often the last straw for a struggling belt.
Good luck.
