for straight replacing Markmon is right on, an MCM woofer with treated paper cone cloth surround are the best bang for the buck I've seen for Db/w output, and really inexpensive.
http://www.mcmelectronics.com/product/55-2951
8-ohm 10", reaches just below 40hz, 95db/w/m, nice specs.
http://www.mcmelectronics.com/product/55-2950
8-ohm 8", 65hz, 94db/w/m, very good output for an 8".

if you want good hitting bass without a powered sub on the floor they'll sure do it.
not knocking the companies making specific speaker kits, most plain jane factory speakers in the backbox can sound a whole lot better just for having some solid bass reenforcement behind them.

YVMW lurch, not kidding about bang for the buck loud, your jukebox should rock! inexpensive but not cheap, you'll see-hear that when you install them. driving a pair of their 12's and 8's with a little modified Lepai 2020A+ is 13w/ch clean, you'd think theres a LOT more power involved, is funny.
(vibrating the ground 40ft away outdoors from 13+13 watts?!)

with small power, speaker efficiency is everything. high power lower efficiency units will just suck the guts right out of a small amp section, distorting before the speakers ever start to get loud, ouch. they can also get an amp hot and cause a failure just trying to get moderate volume from them. its hard to find 4" speakers that'll do 88db or more from the 1st watt of power, if they're good leave them be. efficient stock speakers can suffer for being hit with too much bass power when turned up, is pretty simple to cap out everything below 3-400hz so they dont rattle-distort so badly (bass travel in the same cone, muddying delivery of mids and highs). not saying we expect audiophile quality from a pinball machine's audio tracks and amp section, but sometimes it can be improved a bunch.
-most 2.1 stereos and home theater audio dont drive much bass into the satellite speakers.

if you want to get a little crazier, grab a high level to RCA line level adapter, run it feeding a Lepai LP-838 or 168HA 2.1 amp to drive the speakers instead, gives you volume+tone controls forward, separate subwoofer volume to easily tailor the sound about any way you like (and still be "in cab"). easy to do and the machine's amp section becomes an untaxed cool running pre-amp, oh hey now..
going that route could do a whole bunch more than a simple speaker swap, at much lower cost.
its basically what Spooky has done for AMH's audio, no doubt it rocks, I have a few of these amps.
an 838 wants about 12v 4A, a 168 is ok with 6-8A power supply, 10A if you want louder(?!)..

-definitely something to think about, you can swap speakers all day and never be happy with 3-5w worth of audio power (and lack of tone controls?), or give it the "Tim Allen" treatment AR-AR-AR!
less efficient higher clarity speakers wouldnt be an issue, no playing with L-pads or crossovers etc.