6 inch, 8 ohm, 10 watts like this one:

http://www.radiodaze.com/10w-8ohm-speaker-item-spkr-810pc6/

Heh, I think I might consider upgrading maybe to a coax...

Is there a speaker that is not PinballPro for $90 that would be a little bit of an upgrade for these early SS games?

These guys have pretty good prices.

https://www.sonicelectronix.com/cat_i7_car-speakers.html?affiliate-googadw&utm_source=google&utm_medium=cpc&utm_campaign=Trademark+Category&gclid=EAIaIQobChMI88feyIal1wIVQ16GCh0PtwEtEAAYASAAEgKO5fD_BwE

Remember that you can run two 4 ohm speakers in series to match the signal impedance. This is how I mount the second speaker:

https://images.pinside.com/d/9e/d9e3597514c5a2a12340861a69abc80d3f83d91b/resized/large/d9e3597514c5a2a12340861a69abc80d3f83d91b.jpg

The early SS games sounds are so limited in range that just getting a new speaker similar to the one KenLayton linked above would be an upgrade due to it having a fresher magnet and coil in it. I have just replaced one in firepower, speaker cost £8 and the sound is 100 times better then it was.

As cody_chum said the other option it to go with 2 4ohm speakers, if you do this I would look at your games sound board and google to find its ranges, you may be able to get each speaker to cover different ends of the range thus highlighting different sounds. Depending on how you mount them you could make some sounds (one end of the spectrum) stand out more then the other. Remember though that both speakers will try and 'sound' anything that gets sent down the wire and this can sound crap.