Yes they are 8 watt 8ohm speakers but they're 4" not 6".
I brought some for my Flash Gordon and Xenon locally here in Australia but you should be able to get them at an electronics store or somewhere similar close to you.

Any one will work, you just need to make sure they're flat on the face of the speaker so that they sit flat against the base of your cabinet.
Now that I think about it I have a feeling I ended up going with a 4" 8 ohm 5 watt speaker because I couldn't find a 8 watt speaker. I'd go and check to be sure for you but I'm away on holidays at the moment so my machines aren't around.

FG original speaker are 8 ohms could 4 ohm speakers be used?
How would the wiring change?
Currently + jumped over to other speaker + and the same with -

Would putting them in series work?
i.e. Speaker 1 =S1, Speaker 2 =S2
Do these connections make sense.?
Thinking putting them in series will give the result need.

Positive coming into cabinet feeds S1+
S1- is connected to S2+
S2- feeds negative coming into cabinet

You could daisy-chain/serialize two 4 ohms speakers or use a single 8 ohms speaker.

Most likely the dual combo will give you better sound as you have more acoustic surface.
My XENON has two openings for the speakers but I do not recall if Flash Gordon has one or two openings in the bottom of the cabinet.

Yves

my Flash Gordon originally had two 8 ohm speakers, and looking to replace with two 4 ohm speakers.

Any thoughts?

Serialize.

What was the purpose Bally had in mind with 2 speakers for Flash Gordon? I have Bally games from that era both pre and post FG, and all of my others have a single speaker.

My FG has cutouts for 2 speakers, but only had 1 speaker when I bought it. Sounds fine to me with 1 speaker.