Curtis Youngblood, a world champion help pilot, used to create his own single stick radios for flying helicopters. He did so by taking a potentiometer and mounting it on top of the aileron/elevator stick on the airplane radio. Of course, it didn’t hurt that both he and his dad were electrical engineers!

You could potentially do something similar by taking the wheel off of a wheel radio and attach it to the throttle of a two stick surface radio. I would try to stick with the same brand of radio if possible. In this way, you can still steer with a wheel and have throttle control with the same hand.

If you know any electronics wizards, that would be a big help. There will be some pretty delicate soldering involved.

Best of luck and way to go trying to help someone enjoy their hobby any way possible!