You should be able to find a new industrial 6 volt solenoid with enough pull (load in Ohms same as original). The brackets won't match, and the pull length might be off but a simple bell-crank fixes that.
It also need not be mounted where the original was, or even at the same angle, as long as a good rod links them. I'd fuse it separately since it's exposed to rock damage, etc.
I prefer a cable-operated rod for simplicity (all straight lines use rod, any curves need cable), but a solenoid has the advantage of having a "kill" attached to your reverse lamp switch, etc. A 10" vacuum (Hobbs) switch will prevent engagement unless the throttle is relaxed.
A resistor from 12 to 6 will work, but it's just burning generator output as heat when engaged since it has to be NC or it will drain the battery when parked.
What's actually needed is a small transformer - the loss through inductance (like an ignition coil) is lower than the 50% current loss as heat.
