I would take the MGM controller, it has got some nice features over the 9920 and the last called is rather hard to get at this moment..
You can keep using your 2 speed transmission. that would be no problem.. The size on the 7XL; 25 mm heart/heart for the two M3 bolts to mount it on. the length of the can is 74mm, keep in mind that the wires come from the backside, the diameter of the can is 36mm
About the speed you wanted to reach; your better of with a 4S lipo then!, using the 2speed makes the 15/49 the best gearing for your wish of 40 mph.
Feel free to use
this XLS sheet to calculate the gearing/speed you need...