You are replacing the key switch then?
I have updated a few of these by replacing the key switch. (can use a yamaha electric key switch
I have also replaced the speed controller with 3 micro switches. (resister type older cart)
You can use a regular on/off key switch and then use a double pole double throw toggle switch
for forward and reverse to send the 36v to either of the solenoids for forward or reverse.
The last one I did I sent the wire schematic with the cart.
You can figure it out with a meter. (black wire from meter to battery ground)
Make sure you jack up the cart. you will need to have the batteries hooked up so be careful.
The key switch wires are not hard to figure out.
The forward / reverse solenoids should be on the passenger side just behind the seat cushion.
I am not sure what year you have. Pictures are great for stuff like this.