Your best bet may be to look for a rusted out 7/8 deck and take the drives from it. I just got a deck shell for my 8 but it didn't have the drives with it. You might want to look into other applications that used them. I know Peerless made angle drives that were used in the MTD 960's for the engine drive to the transmission.
I'm about 70 miles SW of the cities in Winthrop. I quickly measured the box and it is approx.3"x4"x5-1/2" not including the protruding shafts. The larger measurement is from left to right in the picture including the "round extension part" of the box next to the pulley. It doesn't seem to have any end play but there is a tiny bit of radial play (a couple of degrees at most). I deck seems to be in pretty good shape otherwise. I just don't ever plan to restore it and install on my 10. What are you planning to do with it??? Just curious.
Just thought of this. I may have one similar on a deck for my White GT1000. I'll have to check it out tomorrow but if I remember correct it has a gearbox on it as well. Another deck I'll probably never install - same price for that one if I'm to sell it.
@jdslednut, Thanks for the measurements I will check to see if that fits. I am going to try and make a PTO for the MF10. There is a small space behind the rear end and the rear plate that I think I can get a gear box in. I am going to try and build a rear mounted 3 point hitch rotary cutter. Just for something different. It won't take a lot of power to run it so I think one of the mower gear boxes will work. If I remember right the ones off the MF7 were smaller and that would work better on the tractor but yours would work and look better on the cutter.