Metal MK8 Extruder on the MPSM in 10 minutes with no major disassembly
You'll want to assemble the MK8 as much as you can. You should wind up with a base plate, a handle, a spring, and some 3mm bolts made to go into the stepper. You won't need the gear and the bearing that come with the MK8. Instead, use the MR104ZZ bearing. It will fit fine. Here are some typical instructions: https://reprapchampion.com/pages/mk8-extruder -- again, fit your bearing, not theirs and don't worry about the gear or the bolts that hold things to the steppers (you'll use those bolts later).
Now, remove all the bolts EXCEPT that one near the dog ear (near the Bowden connector). Now the stepper is hanging from one bolt, which is ok for now. Pull the plate straight up and put it away. The Bowden connector an bracket come away also.
Now take a longer 3mm bolt and use it to temporarily fasten the MK8 base plate through the hole that has a little depression in the plate. This is the bolt that will be under the handle when things are done and you will want a shorter bolt there but not now.
Make absolutely sure that the stepper is engaged on the bolt threads but keep it fairly loose. Do not tighten it down. Once you are sure the stepper won't get away from you, twist the plate as far away from the "dog ear" bolt as you can. The gear will keep you from getting it totally clear. With a little effort, you can take that bolt out now.
Once that bolt is out, move the plate to its proper position and tighten the loose temporary bolt until the stepper is flush with the panel. Now put the two long bolts that are on the filament exit side of the plate in. Once they are in, remove the temporary bolt, and put in the proper bolt (which is quite short).
Now you are almost done. Put the handle and the spring on, and you are all set. You'll need to adjust the tension. Keep in mind that the front panel extrusion is fairly fast and likely to skip. Don't worry about it until you can actually extrude with the computer interface or during a real print. You want the spring tight, but not so tight the extruder skips. Be sure retraction works too.