[Sergio]

I would agree is not a good idea to run those resistor carts for a long time at anything other than full speed.
Another option You may consider is to get a WR586 type reversing contactor and wire it to provide reduced speed.
1) Get the cable that comes from the +36v and connect it to NC contact.
2) Get the cable that comes from the +18v (half-speed) and connect it to the NO contact.
3) Run a new cable from the Common contact to both of the the bottom connectors of the FNR switch where the +36v and +18v were connected.

Wire your new key switch so when the switch is ON, it connects the activation wire of the main solenoid to the activation wire of the contactor.

That way when you have the key ON, the cart will run off +18v.

If you want to keep the +18v for reverse when the switch is off, instead of jumping the 2 bottom contacts of the FNR, just run the +18v to both the NO contact of the contactor as well as the existing connection to the FNR switch.

You can always run a little more speed this way, say +24v or +30v by just changing where the half-speed cable connects to the battery pack.

Later if you upgrade to solid state, or don't need the slow speed anymore, you can buy another WR586 and use both to replace the FNR switch.

Of course this will wear your batteries at a different rate, but I think is better than running the resistors all the time for lower speed.