Do NOT powder coat any engine part that you want heat to escape from
I learn that the hard way back in the 1990s, I had a TM7 intake and set of Cal Custom valve covers powder coated for my bracket BB Duster. I burnt my hands and finger taking the valve covers off after a 1/4 mile run at the old LACR track in the late afternoon
As far as coating steel for corrosion prevention their are several choices like Iraditing (SP?) and several others that have different colors on them like caduim and so on
Some of them can be done at home in your garage
How they hold up to heat and hot oil I do not know if they do or don't