If you used a good spring like from a scale and used a 5 K pot connected to the shaft you could create an input for the Arduino that gives an analog signal approximately for the engine speed.
I though about this to do the same thing you are doing.
But after a while I came to the conclusion it would be better to leave the governor intact and add some springs and a pair of solenoid to increase and decrease RPM to match the load.
I have not worked out exactly how to sense the load other than a shunt resistor on the alternator output. ( that would give 4 steps of speed control not bad I think )
80 amp 24 volt that's a very big unit that I expect will exceed the ability of a GX200 on gasoline to start ( full load dead battery and starting the unit ).
Running would be OK though ( but it is a heavy load for charcoal )
I have a 75 amp 12 dc volt unit for my project ( that I suspect is the limit the charcoal can carry )
I am alarmed about that governor arm being left like that.
I think you had better stop now and remove the parts or do something about this.
I am 99%v sure you have loose parts flying around in that engine ( plastic pieces but still potentially a problem )
If that crank hits the arm while its loose at the right time and place its game over for the engine.
Here is a video that shows what i am talking about with the punch and hammer.
if you notice the minibike its an MB165 and I race those!
I have one that will go 100 kph with ease
Ok I do not race anymore, my back is bad, but I still like to ride now and then and see how fast I can go.
Not too many people want kart engines based on the Honda any more.
But I still build them flat track racing.