I replaced the speaker in two games, williams Space Station and Congo with this pair
https://www.amazon.com/gp/product/B00J2BK2NI/ref=oh_aui_detailpage_o02_s00
They installed easily, cost $30/pair and made a big improvement. I replaced the speakers in the head too
I found some useful info on this thread:
https://pinside.com/pinball/forum/topic/cabinet-speaker-size-and-wiring-williams-system-11
and here:
https://pinside.com/pinball/forum/topic/upgrading-speakers-with-car-speakers