Almost any Arduino can do the job, and you might even save yourself some time and effort by simply using a manually controlled system: robotshop.com/en/simple-slid … =RB-Ins-40 robotshop.com/en/servocity-m … =RB-Sct-51
Note that you will need to install an emergency cutoff in case something goes wrong and the throttle gets stuck.
you can “see” how much rpm the motor makes, if you measure the voltage with a voltage divider circuit to the arduino analog input.
The higher the rpm the higher the voltage from the light coil.
Or you could take a Hall sensor and place it near by the flywheel to count the rpm (there are magnets inside the flywheel)
I am planing to do the exact same thin on my mercury 9.9 hp because always when i wait for a bridge with stationairy motor it stalls when i want to go under the bridge. (i have a mono control) i want to cut the forward reverse from the gas throtle.
maybe electric steering later
btw, i dont think a rc servo is capable of turning the lever of a Outboard motor to reverse and forward (and neutral), so i sugest a wiper motor to do it.