Fixie bike DIY build advice needed.

I have a fixie bike that I am going to motorise. I have a 29cc weed eater engine that I'd liked to install. I am making my own mount out of crutches. I feel that it will suffice for now. It will be a chain or drive belt driven. I will mount the engine above the rear wheel. I do not know if the fixed gear should be used for the engine driven side? Also, i do not believe that there is a clutch on the engine and would like to know about engaging and disengaging the engine. Can i start the engine by pedaling instead of having a pull rope to start.