RE: Engine name
RCGuy:
Going back to the dimensions. The 3 7/8" (3.875" ) is just 0.015" short of 99 mm (3.890" ), so I suppose that could be considered measurement error. The 89.5 mm length from the front end of the crank to the rear face of the back plate is just a hair more than 3 1/2" (3.524" ), but even if these dimensions all match your engine could still have a larger bore than the 0.61 engine.
Bill.