IMO it all depends on the condition of the cylinder/s and whether or not the bore is dammaged as a result of the piston dammage.

The condition of the engine prior to the dammage, should also be a factor in deciding whats the best route to take.

Assuming that there is no cylinder dammage, and assuming that you have an other wise healthy engine. Id be tempted to replace the single piston.

If in doubt. Whilst you have the engine appart it may be a good time to freshen up your bottom end!

Best of luck in what ever you decide.