It looks like a clinton panther go cart motor to me.The color is right,and the cylinder looks alittle big for a chainsaw in the one pic.Then again there were some realy large cc 2 man saws back then too.
I would say the bike is deifnatly around 1945,but the motor is probably about 5 years newer.Great setup,and no lack of power there.Those little Clinton engines are screamers just like the MAC motors.2 strokes don't get much better than that.