For the cost of that 3D printed one, why even bother to give this another thought. It's really not worth the effort to measure and fuss with rigging something up.
You have to practically blow this thing entirely apart to get to the clutch. It would suck to have to go back in because it failed again.
Which does bring up a complaint that I have with the M70, aside from the known issue with the sliders. Fortunately those are easy to get to and remove for repair but I digress.
This radio is the only box I own that I absolutely make sure the tape speed and transport is right with a day or two of playback before I put it back together. The reason is, I don't want to have to remove the chassis again for adjustment or repair. It isn't too hard, it's just more than I feel you should have to do. Especially when it comes to adjusting the tape speed. Unless it's the model with pitch control, it's almost worth drilling a tiny hole next to the counter wheel on the front and putting a M70 decal over it.
I'm sure the idea was that you shouldn't have to touch the speed after it's adjusted correctly. In those days, that made sense. However, the crappy or NOS belts that are past their prime today, tend to stretch after a few hours of playback which would make a convenient speed adjustment welcome.