If the Razor MX350 dirt bike is a version 1-8 then its stock throttle is variable speed type and can be used with any of the 36 Volt controller that we sell. However the battery meter on the MX350 version 1-8 throttle is for a 24 Volt battery pack so it will always show full on a 36 Volt battery pack even when the pack is empty. The battery meter could be left disconnected to disable it though.
The MX350's stock 24 Volt motor can be run on 36 Volts and its power and speed will increase due to it being overvolted. However when ran on 36 Volts it will be prone to running hotter than it does on 24 Volts so its temperature should be monitored during the initial runs of the completed modification to make sure it does not overheat. Voltage does not burn out motors - heat does. So as long as the motor does not run too hot on 36 Volts then it will be okay.
Please let us know if you have any further questions.
That controller looks the same as our SPD-36800B controller and we have wiring directions to it at this link: http://www.electricscooterparts.com/hookup/SPD-36800Bwiring.htm
The new 36 Volt 800 Watt motor might be able to be mounted above or in front of the rear wheel if it is to big to fit under the frame.