I added a Magura brake to my muni and bought a kit of parts to do it from brycer1968 here on the site. I haven’t seen him on the site in a while and he might or might not still have parts to offer, and you might or might not want to purchase from the States. Either way you might learn something about what it involves by looking at some of his listing pages:
As for mounting: I had experience with several in-between sizes before trying to mount a 36" but my guess is that it would be a big change going to that from a 20". Just about everyone jumps well enough to get onto a 20" by just hopping on. Not many people have the vertical capability to get onto a 36" the same way, so it takes some degree of stepping up via the back pedal and learning how to do that without rolling the wheel out from under you. There are countless discussions in various topics but generally it takes better control and also more physical effort than mounting a smaller wheel. The first mount of the day is always the hardest for me, at the start of the ride before I’m warmed up or tuned in to correcting my balance.