I'm currently in the middle of making one of my own off of this particular Mauser 712;
http://www.evike.com/products/24054/
It's not impossible, and if you aren't too concerned with screen accuracy its not a bad base for it. The firing on the gun isn't too bad, the gas is indeed housed in the magazine with the BB's. The only issue I had with it was the color that it came with. The gun was this dark gray color that I wasn't too fond of. I stripped it down and used some Birchwood Casey's steel blue and I really like the way it looks now.
The firing selector switch is removable but it leaves a hole and slots in its place. These can probably be filled or brazed and blued, but I didn't want to try that due the gun body probably being steel and I only have sticks of Aluweld or whatever it is called. The milled slots on the gun itself are not totally the same as the c96, they are a bit more idealized to fit the gun body.
As for the magazine, it can be cut down, but it was not an easy task. There are pins on the side of the magazine housing that keep the base securely in place. Once these are popped out, I went at it with a hacksaw, cut it down, and smoothed it out. Unfortunately, the base and its gasket requires a bit of a recess to fit back in, so I had to mill out a pocket for it to fit back in and cut the spring down the corresponding length. I surprisingly enough managed to do it without any leaks (as far as I can detect) but the gas capacity is significantly reduced. Not that I mind, I don't plan on taking it out of the house at all or using it in any situation which would require a lot of shots, but it's something to keep in mind. I have seen short mags that are made for this model, but everywhere I go its sold out or unavailable. Actually, here is a shortened magazine, I may have to get one just as a backup.
All the parts I machined myself based on the paper robot drawings that I found here:
http://www.paperrobots1999.com/downloads/DL44_ANH_rev9.jpg
The barrel can be easily cut with a hacksaw, as can the inner barrel that the bb's shoot through. Dissassemble the gun, wrap the inner barrel in tape and reassemble and it will be perfectly secure.
Since I'm not a machinist by any means, I can't be too worried about screen accuracy (because I can't machine something within tolerances that close lol), It's not done yet, I just have to finish the scope mount, reblue the scope and mount and finish weathering everything. But long story short, it can be done.
EDIT: And as for the scope mount bracket itself,in order to ensure that there is no interference with the internal mechanism, I used JB weld and some standoffs, I saw someone do this on a real Mauser, and it has worked well so far, except it fell on the floor once and popped off, I have since re glued it back on, and if it happens again, I will just have to braze it on.