I have an 1989 325I with a broke cam belt, so the valves are bent. Can I just have the head rebuilt and put it back on the block or is it possible that the connecting rods are bent also? What do I look for on the pistons to determine if they need replacing?

i haven't run in to that problem with my 325 yet but check the pistons for chips or cracks if thet are replace them . as far as the rods it takes alot to bend them if the pistons are ok just have the head rebuilt

I have had that trouble on race engines.

Remove the pistons and make sure that the
rings are free and the pistons are not damaged.
If not you can put it back together and go.

I assume your going to replace the piston rings as well. If so, the lump would benefit from having the bores honed out. It'll mean running her in again for a while but if she's vintage it's worth it !!