With a little modding the bigger guys will work
The 120-160 1s should be fine. The 160mah I have here are slightly bigger (longer) than the 120mah. The 160mah are listed to fit the MCX, and from pictures of the MSR all you need to worry about is width. So go with anything below 160 mah and should be fine.
P.S.: Also looked up an MSR review
and video...width is the only factor. the 160mah WILL work
If you still want the baby 120mah I have one or two around here.