I figured it out, thanks to ArcadeShop! It's a 6x9, 8 ohm speaker.
While I'm at it, I can mention that I rectified the fuzzy screen problem, check! I took off the marquee to see what was going on with the light. As it turned out, the connector wasn't even plugged in! So, I plugged it back in, cleaned the filthy bulb, and now we have light!
So far, so good! I just ordered the new speaker, I'm now waiting for some coin door lights and a Galaga manual that a local collector friend is hooking me up with, and I'll replace the old levelers eventually.