80 watt RMS. I don't know anyone who managed to get a more powerful amp into the space under the rear fender/mudguard. If you can crack that nut it would be good to know. The few chaps I know who upgraded their amps put them in the trunk space for the CB radio, which is fine if you don't have a CB.
Here is the stock amp wiring and also removal instructions, if they are of any help.