I’m assuming that the chair has cycle type rear wheels (as you mention ‘like pedal assist’) and you’re looking into replacing one of these with the hub motor type? If you replace only one you’d need a connecting drive rod between the wheels, I’ve a single hand drive chair and it’s just a sprung square tube that you pop out for collapsing the chair. Firstly I’d measure the wheel on the chair and then Google to see if hub motor wheels in that size exist, and then I think I’d see if there’s a decent independent cycle shop with experience of doing bike conversions and talk the possibilities through with them?
It is an interesting idea though, especially with a discreet throttle controller for electric only drive.