Prophanger, I came from an ariplane background and it took me a while to believe that the "little" .15 t-maxx engine could have any kind of power let alone as much if not more than my larger airplane 4-strokes, but they do. They have much more rpm and as such the trucks are geared for it. a four stroke would move the truck but you won't go very fast unless you make major gear changes. You won't be competetive in any race. As far as brushless... they are not legal in most any race unless specified. were I run I have not seen any brushless class yet, but that may change....
Lastly, you definately belonged to AMA. unless it is different at your track, you can run in any class you want. You don not have to be certified(approved) to run a certain class,but please practice on an empty track or during practice time to get better before banging with the other guys. (nothing worse than taking out an expensive truck because you can't drive)