You may not be able to find specs for that particular transformer. You can try searching for its model number, manufacturer, or whatever. I've seen a site with info on A500 power supply repair before, but that was some time ago.
The things to be aware of when replacing a transformer are secondary output voltages and current ratings. The secondary voltages will then most likely be rectified, filtered, and regulated. You should be able to find data sheets on the rectifier diodes, caps, regulators, etc., and you can work your way back to the sinusoidal voltage from there. Do you understand RMS voltage ratings?
-Oli
edit- Your PSU may work acceptably well for some time to come. You just have to judge how bad it will be if your machine eventually runs on lower than rated power, or experiences power failure(s).
