I found, I usually take pictures of these back of the napkin ideas.
You can use the existing 3-tab reverse micro switch and existing 5.1kΩ resistor if it is in good shape.
You will need to buy a 2.5kΩ x 1/2 watt resistor.
To buy online both online:
2.5kΩ Resistor
5.1kΩ Resistor
That wiring will convert the throttle from a 5k-0Ω to a 5k-2.5Ω signal when you switch to reverse.