Taken from Zippys
Most 24" or 26" bicycle with diamond frame and horizontal top tube should have sufficient space to mount motor.
At least 12.5 inches running along the seat tube from the top of the bottom bracket to the bottom of the top tube.Tubing size from 1-1.5inches on the both the seat and down tube. Plus or minus 70deg. above the bottom bracket between the down tube and the seat tube. Inner distance between pedals needs to be greater than 4.5 inches in width to clear the 4.5inch wide motor.
Instruction Manual - Gas Powered Bicycle Engines, Motorized Fast Bicycles, Motor Gas Engine Bicycle, Bike Conversion Engine Kits, Motor Assisted Bike Engines 80cc and 49cc, Peddle Start Motorized Bicycle Engine and more!