Just bought the last $10 cannon toy on ebay from a particular buyer.
http://item.mobileweb.ebay.com.au/viewitem?itemId=320858057820&index=2&nav=SEARCH&nid=25811902107

The length is too long but I will try and cut the front barrel part to sit on rotating mechanism and the rear legs to be fixed above the right sling.
The left wheel should be OK as is because it is the ideal diameter of 2.5 inches. The right wheel will need to be fixed and probably modified due to wiring and space restrictions.
I like this toy because it is light plastic and will not cook the gun motor.
If this works, it will probably go down as the cheapest ACDC mod.
If it does not work, I will not cry over $10.