When i had my mini t. i put together a 6 cell battery with AAs from the store that were 2600MAH. Then i bought a Beast Raptor from one18th.com's store when it was open. If you talk to the right person you can still get that good motor, and made a nice batt pack for your son, and it will be very cheap!
Like most aftermarket motors you do need to do some small work on the stock motor mount. Other than that its a great upgrade to really make your kid happy