It certainly could be done without too much trouble with the drivetrain if you're willing to have a driveshaft built, etc.

The problem in the case of the 126 body will be clutch and shift linkage. There are VERY FEW 126 cars in the US from which to salvage these parts. It's possible that the pedals, shifter, etc would interchange from a 123 car, but I sure wouldn't enter the project while counting on that.

I typically say that I would not enter into a project like this without the donor car setting next to the recipient. In the case of this project that will most likely be impossible.

If you are determined, you need to PM leathermang. He has a source in Eastern Europe for salvage parts and may be able to point you toward a source. The 126's with manual trans are not uncommon in Europe, it's just that none were ever imported to the US except as grey market.

Good luck,