Difference in drr frames

My son is 5 and has been flat tracking his polaris scrambler 50 and loves it. I am wanting to get him into something he can be a little more competitive on and can find parts for. I was wondering what the difference was between drr 70 motor/frame and drr 90 motor/frame? Just wondering in that if i get him a 70 can it later be upgraded with 90cc top end or am i gonna be stuck with a 70 that he will grow out of much sooner? Any advice is much appreciated.
Thank you

The frame between a 70cc and 90cc is the same but the engine cases are not so you would have to buy 90cc engine.The 90cc engine will bolt in with no issues so later on you just buy a 90cc engine and bolt it in.

If your looking for a new Drr www.rectires.com is the best. Everyone else will charge shipping.