Bruder Mack Truck poison motor

[Themonty73]

Hi guys please help,

I'm trying to convert a bruder Mack truck to rc for my step son, I've put a driven steering axle in and this is great but I have attached a poison motor to it and it goes to quick, I've tried to regulate the speed but to no avail I was thinking of converting a servo to 360 degree rotation to slow it down a lot but not sure would like some help on this please.

Thank you