The lady who bought my '89 325i took it to one of those "we can fix anything" gas stations when the water pump(driven by the timing belt for those who don't know) started leaking. They put it together more than one tooth off, and it bent several valves and wouldn't start.

She had it towed to shop where I was working at the time, and we ended up pulling the head and replacing several of the valves and guides(they cracked when the valves bent).

It was also a good time to update the head bolts, the OE head bolts are known to have the heads break off without warning and the loose piece eventually gets under a camshaft lobe..... very expensive at that point.

A vacuum gauge will halp you diagnose the bent valves, a fluttering needle is a good clue. I wouldn't pull the head until I knew for sure that the valves were bent, even though it's possible it happened.