I think I would have gone with a good quality DC motor driving the original speedo, a piece of vinyl tubing makes a decent low torque shaft coupler.
I read in some article that these strip speedometers were notoriously unreliable so direct drive would let me bypass most of the mechanical components that may be contributing to this reputation.
If i were to drive the speedo cable with a motor i would not only add a wear item, but it would need to be constantly running and whirring in the background. I would also be trusting a mechanism from the early 70's to be operating accurately and reliably.
And i don't think that's a good solution.
If you want a more direct approach, how about taking the electronic speedometer out of a newer car and transplant the mechanical bits behind the existing faceplate? With a bit of luck you might even find one that has the odometer in the same location. IIRC the speedometer in my Volvo is essentially a moving coil meter.
I think that's a bad idea actually. For starters what i have is a strip speedometer and i don't think there are any electronic strip speedometers around.
I would then have to shoehorn a dial speedo in, which would ruin the aesthetic (that retro strip speedo vibe) and before that i would have to find one that even fits and looks right.
After that i would have to somehow reverse engineer the interface for said speedo and chances are it wouldn't work with my gearbox/ gear ratio combo so i would need a custom interface to convert the signal to match anyways. So i would be making some kind of control board brainbox regardless.
So basically i would be doing the same thing except it would be more work, more convolution, less reliability (imo) and the result wouldn't look as good in the end also.