Some insights. Recieved my junk original motor. In comparing the Solex/Hongdu, there are differences, close but not the same. The original piston is ported differently, different timing and the holes are bigger, intake port in the cylinder is slightly bigger on the original. The piston in the HD is slightly larger, wont fit the Solex cylinder. The rings appear different, the original appear to have more cross section(were stuck could not measure). So they have made it difficut if not impossible to mix some parts. The external parts fit, cylinder fits also. Have not yet checked the rod pin to see if the rods will interchange. My original thoughts were to run an original Cyl/piston/rod. So, if replacement parts are needed Steves Moped is the place, he is selling the BlacknRoll/Velosolex parts, which should be compatible. VeloCruz I believe sells qriginal stuff. Also if the piston pictured in Steves parts listing is what you get, then it is like the original but for thr HD. So it appers there may be 2 versions of the engine, ours is somewhat de-tuned.