i have a version 7 sti engine but 2 valves have broke and put couple of dents in the piston what else will need changing ? could it of bent a conrod??? is it a easy job to change the piston and where will i be able to source one???

 

full strip down, may as well replace the bearings as well etc. I personally would change the rod, doubt that it would be bent however it has had extra stress placed on it. Maybe a good time for uprated internals.
What caused the valves to hit the pistons?

 

The following are all possible, with varying degrees of likelihood, but you should check everything

Bent Rod
Flatted big end shell
damage from debris to the cylinder walls / and or piston / rings
Cracked valve guides on the affected cylinders
Damaged valve collets and retainers
Cracked buckets

I have seen all these on engines that had apparently just "had a valve clip a piston"

 

the car the engine came from had light front damage but never touched the engine so my guess is the belt slipped a couple of teeth causing the damage im not to boverd about the damage in the head because i will rebuild the short motor and keep as a spair for my version 2 sti

 

to be honest for the belt to slip a little, the engine must have been touched. Like Mark says there could be possible cylinder damage

 

The result of a Cam Belt Tensioner retaining bolt shearing

 

Looks like a country waterside scene with bull rushes waving gently in the wind