FYI, I did finally get the Rockfire adapter below and tried all modes and found Mode 3 is best. Only downside is there are 2 buttons on the joystick but they aren't 'mapped separately' so I'm firing & bombing/using shield at the same time....but it beats nothing!
https://www.amazon.com/game-Adapter-Rockfire-RM-203-gameport/dp/B004HAX7OU