R-R used to totally immerse B & K range grey iron c/case castings in paint and bake the c/case before machining. I recall the process was called S O, no idea what it stood for. The main purpose was to safely seal any residual casting sand in place. It was very effective, nothing seemed to remove the paint, which was a sort of silvery colour.
For your c/case, assuming it's clean, do nothing other than use traditional Bluecol antifreeze at a 50/50% mix, as it contains adequate inhibitors that will prevent further corrosion.
If you're determined to paint the internal coolant c/case area then you'll need a bath full of appropriate paint or a specialist contractor, both won't come cheap!
If the MCC engine is rusty internally I'd check if it has a water tube running in the c/case from the water pump distributing coolant to each exhaust valve. Nasty things, a sort of zinc plated tin tube and very important to keep the exhaust valves cool on a side valve engine.
Good luck. Jerry