Phew. The main challenge will not be the electronics, but the mechanical part. To make this thing half reliable, the mechanics must be built to reasonable tolerance esp. regarding the fittings, and weatherproof. But OK, assuming that can be handled:
1. Why try to replicate the original mechanical setup at all? The reason for this rotational gauge contraption is the lack of "intelligence" in the system, so you must convert angular speed to angular displacement. Why not just discard all of that and use the stepper directly? In the latter case the stepper if perfectly suited to turn the speedo needle this way and that under simple program control.
On the other hand, if you find a stepper with suitable specs (torque!) and can fit it on the existing input coupling, then why not replicate the original too...
2. You need to implement a stepper driver in every case. While there are many ways of doing this, i am still going to recommed the combo L297 / L298. The dual full bridge chip is probably overkill as far as power levels are concerned, but this is a simple surefire way of getting the stepper part to work. The L297 is the step logic generator and the L298 is the full bridge and those 2 interface directly. The 297 takes simple step/dir input commands making control of the stepper real easy.
3. You could use (gasp) an Arduino to do the conversion between the hall input and the stepper control output. Although if it was me, i would just grab a Mega328 and implement the board directly. No big deal.
4. The software is real easy: time the input pulses and generate a pulse interval modulated output based on the multiplication factor programmed into permanent memory. You could even implement a signal conditioning curve if the input (or rather the gauge transfer function) is nonlinear.