Why not cushman pistons they are the old standard and allow 74 wheels and rods?
If you insist on modern slipper skirt design ck out Datson pistons, I think that the one that is used is from a L20B but memory fails. Wichever it is it has to be trimed on the skirt and will require 45 rods.
There is also suposed to be a Ferrai piston that will work. But I never checked this out as the price is bound to be incredable.