68.5mm short rod and 20cc cool head, Cylinder is ported by sand trix. Does it need race gas?
Rod length, a 68.5mm bore, 20cc dome, and who it is ported by does not really give us the type of information we need to advise you on what type of fuel you need to run.
What is the piston to head clearance. What is the volume in the combustion chamber when the piston is at TDC. Show us a drawing of the combustion chamber. Show us a port map. Tell us what pipe and silencer you have and what the stinger inside diameter is, then we can advise you on the fuel to run.