2 parts to this question... what's the best way to clean aluminium fins on an air cooled engine? (and protect once cleaned?)

The exhaust ports have very heavy carbon/soot build ups (diesel engine btw) I don't want to attack it with tools yet as I don't want to damage the aluminium beneath it, so is there anything I could soak the head in that will break down the build up without attacking the aluminium?

There are two cylinder heads, each one is small (about 20x10x10cm) so I won't need a big container to put them in and will only need a few litres to completely submerge them.

my old chap swore by soaking in diesel

Carb/ brake cleaner and give it a soak. Soda blasting on the fins.

Injector Cleaner soak?