I would imagine that the Sport Trainer with the modifications to
include a 0-320/290/235 and a pressure cowl would be closest to the
Supercub, but what you really have is a PA-11. The main spare meets
at the top center of the fusalage in the J-3/PA-11/Wag Aero Sport
Trainer, instead of outboard like a PA-18, so the spar configuration
is different If the 0-320 is installed in the Wag Aero, additional
bracing has to be added from the spar attach point down to the
fusalage. That's four tubes between you and the windsheild. If you
want a PA-18, start with a PA-18 airframe.