Do you mean a rack battery? A photo would help us understand what you have. Motors are normally quite reliable and there are ways to get a lot more power out of them. Do you know why yours stopped working?
If you've been looking at those 500w kits on Ebay, forget them. They won't give you what you want, though there are other 500w motors that might be OK. The power comes from the battery and controller, so sticking a 500w motor in the back won't solve anything. you'll need a battery and controller to go with it. You'd be looking at about £500 to £600. If your present motor is OK, a 48v battery and controller would give 33% more power for about £300. If it's just your controller that's faulty and your battery is still good, about £60 would get you something like 20% - 30% more power.