Question 1: No the 20T Axial Motor is not rebuildable.
Question 2: I suggest you just upgrade the motor becuase the stock ESC is perfectly fine and is very capable. You don't need to spend the extra money on another esc if you go the brushed motor route! A brushed Tekin 35T is pretty good but it won't give you alot of speed. You can go with a brushless crawling combo from Novak but only if you DON'T go through water. Brushless motors are not waterproof! The brushless is more money because you have to buy the ESC too. and because its brushless.
FYI there is no such thing as sensored in brushed motors.
Hope this helps!