When rims are used with spokes of different lengths on each side the hole in the rim is drilled at an angle to suit so this rim would not be suitable. I dont know how the spokes are arranged on an M20 so cannot help.
The best answer would be to treat the old girl to a new rim with the wheel built by an expert as this is not a DIY job
Yes it is
I've built mountain bike wheels, had them checked by a master builder with a clean bill of health. It wasn't a difficult job, oh and I did some motocross wheels too, that was easy. And I'm not particularly gifted with my hands