At least covers shoud be removed and polished with a maschine.
You can’t get great results as long as covers are still fitted.
Think about new gaskets when removing the covers !
You will need to start with coarse grinding paste to get oxidated surface removed and then go on with ever finer polishing means until they shine like chromed !!!
Takes a while and is a dirty job !
Polishing of motor covers is sometimes as a service and price is approx. 100,- Euros per cover !
It is still pure aluminium and that tends to oxidate again.
After a good polish surface is “hardened / more resistant” to oxidation, but it will be a regular job for you to polish the again and again and again (once or twice or … per year) to keep them in nice condition.
A heat resistant coating / painting with transparent color would help but it is not easy to achieve. A specialised company should do that job !