There is a long history of using pistons from a different type of engine, for some particular purpose. At one time there was something of a practice in Australia of fitting Ferguson tractor cylinder barrels to Triumph TR2 and TR3 vehicles, together with ordinary Chevrolet pistons, to increase the displacement from 1.6 to 2.6 litres, while raising the compression ratio to something approaching 10:1. One of the attractions of this change was that it was sometimes cheaper than fitting new standard-size barrels and pistons. (This odd-seeming transformation was only practicable because the 4-cylinder TR Triumphs used the same engine as the Ferguson tractor of the same time period, of course. The engine was also used, in much larger numbers, in a rather unlovable car called the Standard Vanguard.)
Essentially though, in this case I doubt that you want to over-bore your engine to the size of some other piston, or alter your compression ratio by using a piston which has a different compression height, or grapple with problems relating to differences in gudgeon pin diameters. You also obviously face some extra restrictions due to your engine being a port-controlled 2 stroke, so you require piston ring pegs, and a skirt height identical to that of the original piston, since the piston skirt height controls the intake port timing.
If you can contact a group of JAP engine afficionados there is a good chance that they will have already investigated whether some other piston can be used satisfactorily. That does not mean they will have found a feasible solution, though.
