It would be cheaper to buy another 123 with electric windows than to
convert your car.

Each door needs a "regulator" that's an electric motor and drive gear,
the wiring harness that powers the motor, relays to switch the power and
finally, the console and rear door switches and the wiring harnesses to
connect everything. One can buy the regulators used but the wiring
harnesses, relays will be new and relatively expensive. Then there's the
labor and the risk of a mistake damaging your car's electrical system.

Cars can be repaired relatively easily - remove and replace the broken
part - but quite difficult and expensive to modify as you propose.

I'm sure somebody's done it. Go to a wrecking yard and procure
a 123 door, complete. With glass and interior panel and electric windows.

If you're lucky or patient you can find one in your paint color
and the same interior color. And no rust. NO rust.

Now bolt this door onto your car, run a few extra wires and you're
done.

If the door is not good enough or you only get the parts, then take your door
apart andd add the moto. You will need ther interior panel from the auto door
if I recall.

It's good to have the other door as parts. They'll be more useful as time goes on
and good doors get hard to get. It's not that easy now in some areas.