Bicycle weight limit (carrying capacity) increase
On a non-folding bike,  I'd look at the wheels, and probably replace them with 36 spoke touring wheels and reasonably wide, tough tyres (e.g. 35mm). The tyres absorb some of the shock loads.
However a folding bike has made some compromises affecting the structural strength, as well as introducing weak points. Two major ones are the main hinge and seat post. The latter is probably OK as your added load isn't on it, but the main hinge will be under added stress from the extra unsprung mass. It's probably not replaceable or serviceable, and obviously overloading the specifications would give the manufacturer an excuse to get out of warranty repair. The risk here is that the frame won't last as long as it should.
Adding a mid drive motor may not be as easy as you think - there's not much room given the need to not affect the folding, combined with the small wheels (short chainstays and reduced ground clearance). A kit for a different folder might help, but might be very specific in its mounting.