Rob,
The Forney has a standard layout of throttle in the center and Johnson bar on the right/stbd side. The usual solution is to bolt a servo in the coal bunker/water tank area connecting with a long rod to the Johnson [reversing] bar. You'll need to remove the pin and spring that slips into the indent.
This is a Ruby, not a Forney, but they are essentially the same:
View attachment 349219
Here's another pic of a reversing lever and servo on a C-16:
View attachment 349220
This is my old forney with the cab off [I fitted a wooden cab from a kit.] Lots of room in the water tank.coal area (red box) for servos, batteries, and the receiver.
View attachment 349221
The throttle servo must be bolted below the throttle arm at right angles, so the servo actuator is working in the same plane as the throttle. If you have a round knob, like mine, you need to get a metal RC arm from Accucraft or Servocity.
Then you connect the throttle servo to the throttle. You'll find that only 90 degrees of movement of the throttle will be adequate. My first photo shows a typical Ruby configuration, which will work in your Forney.