if the bearings on a motor bike are worn and been replaced will i have to replace piston kit too?

Answer 1

Depends on the wear pattern in the bores. If there is a ridge where the top piston ring reaches the end of its movement then you need to make sure that the ring isn't pushed over this when the engine is re-built. There are special rings with a cutaway edge for this problem. Ask about a "ridgedodger". The new bearings will of course push the piston up a tiny bit further than the worn ones.
If you've stripped it this far then it would be a false economy not to re-do the top end as well.

Answer 2

If you have it that far apart you would be foolish not to change it.