Can I replace the factory installed common wire on a GFCI circuit breaker?
What a fool. He owes you $250 now.   (although I don't see that breaker costing anything like $250 unless Canada is somehow involved.)
You could try getting a 2-void Alumiconn lug connector, that only requires you strip off about 3/8&quot; of the wire length to go into that.  And then pigtail from there.  This is allowed in panels (unless Canada is somehow involved).
You are probably better off just replacing the whole pool panel with some other brand which has cheaper GFCI breakers.