Derren Nesbitt would have been a good Hackman replacement.

British actor Derren Nesbitt specialized in German and other European characters, most famously appearing in Where Eagles Dare as a German. I think he could have been quite convincing as a Pole. Jeremy Kemp, who did appear in this film as a Brit, might have worked as a Pole also. Max von Sydow might have worked. Although Swedish, he probably would have pulled off a Polish accent better than Hackman.