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Hi Roger.
Yes, your Scott should be perfectly happy on unleaded petrol. As you seem to have licked the overheating by advancing the timing but are only firing on one cylinder, the next thing to test is the crankcase compression. Take out both spark plugs, and earth both plug leads to protect the magneto. With the petrol turned off and the throttle wide open turn the back wheel slowly with the bike in 2nd or 3rd gear. As you turn the engine over you should hear a loud “plop” twice per complete engine revolution if all is well “down below”. I suspect you will only get one Plop per revolution, in which case one of your cylinder base ring gaskets has probably blown. Another but unlikely possibility is that one of the glands is not seating properly. This can be tested without dismantling by filling the suspect crankcase side with some oily paraffin (kerosene). If the gland is not sealing properly the paraffin wii leak out adjacent to the flywheel sprocket(s). Good Luck!
Regards and best wishes. Brian.