Will a gas motor fit in a T-Maxx? Yes and No.
It has been done but a LOT of very major mods are needed. So, yes, it can be done but no, it is definatly not an easy thing to do. If you would do a search on "Maxx Gasser" I think, you will find a very long thread on one that was done.

But he is right about the tuning, they require very little in the way of tuning.