If you are getting the battery for free, then take it, otherwise its gotta be real cheap to gamble on it.
Is it a Lithium Ion or LiFePO?
From the looks of the Goldenmotor.ca website, the 36V are rated for 30A ah but I did see one that was 25A, so yeah 900W-1080W is fine. 10 cans in series and then 4 in parallel. If I am not mistaken the controller is inside the motor, you can use an external controller if you wish. Any of the ebay 36V controller would be ok.
If you can, open up the pack and see what 18650 cans are being used. Maybe its just a matter of tearing off a bit of the heat shrink from one end. Or opening up the hard case. Its better to do that anyways so you can inspect each can and the bms.
Take lots of pictures and post them up here and we can help you!