Mazb,
I did mine so it would be easier to set with my finger. When I considered using a servo I was gonna either route the flexable control rod from the other side and come across the firewall or put in a bellcrank similar to my throttle. Or both.(Brison Engine)

The first option I mentioned would allow you to use the servo mount on the other side of the tray.

I didn't want to modify the arm in case I want to sell it in the future and I have an optical kill switch for a positive way to shut down remotely.