I think it's just a 12V solenoid that engages the unit. Could probably run it with a switch and/or relay directly without all the electronics. The issue is it has to disengage every time you slow down to less than 10-15mph to avoid damaging the unit. It would be easy to get distracted and forget about it. Is it worth damaging the unit?

I recommend just getting all the electronics, speed control and switches to make it operated as designed with a automatic or manual mode. These items can be found on ebay from time to time. I have one in a 68 dart with 4:10 gears and love it.