its worth thinking of the fact its just an old pc really.. any AT power supply that fits would likely do the trick.

what he said.

I used an old ATX power supply from a junked PC that was lying around after my RFM one died - you may have to do a tiny bit of wiring modification as it will be unlikely you can track down a AT power supply, but an ATX will work 100% if you ground the green wire. For originality & perfection, you can wire up the green wire to the switch on front of the PC box so that still works, though you'll never need to use it.

Great guide here - http://antelopearcade.com/forum/yaf_postst4_Pinball-2000-AT-to-ATX-Power-Supply-Replacement.aspx

IMHO, leave the replacement of the power supply until it actually dies as it's unlikely to cause any problems when it does die, BUT replace the CPU fan ASAP - if/when that dies, you run the risk of cooking the CPU. Try and find a decent fan that has proper bearings - jaycar should be able to help.