Actually I have moved away from the pusher to the Rhino friction drive... There is a motor that you can buy for thirty bucks from all electronics, the link is around her somewhere that is 600 watts but can be run at 900 watts by adding an extra battery. That one is a good deal. Actually the best deal I have seen.
If you look on ebay now and then you can find a schwinn 350 or 400 rear wheel from a scooter. It will have a motor mount that will work with the motor above. The mount allows the motor to hang from the pusher axle, but if you wanted you could also have it hang from the bike wheel. You would need a sprocket on the bike wheel if you did that. The friction drive is easier.
That should give you an idea. If you want I will give you a set of plans for it. I have been meaning to write one anyway. I have never built an easier friction drive. I use a controller and throttle and 36volt battery packs.