I agree with RadioRon.
You could replace Q2 with a N chgannel MOSFET, but you would need to calculate the power dissipation.
It is a crude circuit. If it were mine, I would modify it so that Q1 is turned off when the brake is applied.
A simple way to do this would be to replace SW2 with a SPDT switch and connect in the +12 ~ 16 Volt line so it turns off the power to the accelerator and applies it to P2 for braking. But I know nothing about model car controllers, so I may be missing something.
I don't understand what you mean by "as somtime you can fell the delay from a realy"
Obviously realy is a typo and should be relay, but what does "fell the delay" mean?