Motor control of resistance on exercise bike

Hi all – I’m looking to add motor control to the resistance knob on my exercise bike.

The resistance is magnetic, so little effort is required to turn the knob.
I’m not looking (at least initially) to be able to dial in a specific resistance, just would like a predictable way of adding or removing a set amount of resistance.
I’d like to maintain manual control of the unit through the knob, and want to build a solution that doesn’t require modification of the bike.

Right now I’m focused on the interface to the resistance control.. do I do something belt-driven, or might a roller against the rod be sufficient, perhaps with some tape around the rod itself?

Suggestions most welcome, including hardware for building a quick and dirty mount.. then ill work my way over the actual control..

Thanks in advance! Joe