To start, I know little to nothing about ground vehicles models anyway.
If you are happy you have eliminated the state of the battery the only 2 electric options left are the motor and ESC, have you tried swapping these out if you have access to spares.
Ahhh...... make that 3.........also check/re-do all the solder joints, I had a dry solder joint on my 450 heli and could not get enough rotor head speed.