What engine are we talking about? My advice is to find buy this kit
http://www.2cviking.eu/catalog/product_ ... cts_id=568
or just find the pistons you need. Unless you know, I would ship off the heads to an engine rebuilder for total reconditioning. When all back together, you'll know that the job is done correctly and no more opening the engine for a while.
Valves may be old and worn (out) exhaust stems are no good after years of heating and cooling. The metal is weak and changes, you can adjust the valves every 10.000 click
No don't bother, you can fiddle all the time and may regret that you didn't
Cheers
Viking bastard

PS. Sometimes we need to except that parts do not last forever and some can not be reconditioned.