Your sketch will need to detect whether the engine is actually idling. Typically, idle control valves effectively held wide open during ordinary driving and deceleration in order to avoid high manifold depression. But if you aren't concerned about emissions and EGR you could ignore all that and just accept that the controller will have the ICV pulled down against the stop trying to drop the revs most of the time. You might find that causes problems with the ICV sticking and clogging, but there's only one way to find out.
Today, the engine is run with a fixes bypass to the trottle, so i can not se the need for having the valve open.
The car has no EGR, so no problem there either.
Depending on the engine's response, you might find you need to catch the revs softly as they drop down to idle rather than let the engine spin down with the throttle closed - some engines don't respond well if you leave them on overrun and then suddenly ask them to idle.
Only one way to find out.
You'll need to tailor your PID settings to match the engine's throttle response and to make sure you aren't tripped up by a lumpy idle. You need to decide whether your idle controller will respond faster or slower than the engine. You want to avoid having it respond at similar speed to the engine as that will lead to hunting. Probably, you'll find that controlling the ICV slower than the engine response is easiest to get working i.e. think of it is primarily an integral controller. I think your best bet for that would be to find out what ICV position corresponds to a normal hot fast idle and park the ICV there until the revs seem to be dropping in to an idle position, then make sure you drive the ICV closed slower than the engine will respond
I think i can figure out the settings for the PID on my own, has some experience about PID from school.