I took a look at the aircraft.cfg and panel.cfg of my Waco and all is not lost.
First, there is a section for the autopilot in the aircraft.cfg. To activate it I would change autopilot_available= 0 to autopilot_available= 1 and add various parameters from another aircraft - perhaps a comparable aircraft or the default Cessna. I tend to just use the shortcuts to activate the autopilot, altitude and heading anyway (Z, Ctrl-z, and Ctrl-H).
As for the gauge swap, there are several radio gauges scripted under Vcocpit01 and Vcockpit02. Not sure which is which or which could be replaced, so I would do some experimenting. Just add // to one gauge at a time, load the aircraft, and see which gauge you want to replace. If you find which one references the GPS, then just replace the gauge...