Zinc coatings needs to be pre heated before the powder is applied to drive off any gases or you need to use an anti gas powder. You cant get the stain off.

Pictures Indy Powder Coating
Powder coating an engine. These cast aluminum parts can really dress up your engine bay once powder coated but they can also be very difficult to get 100 clean when you try to powder coat them. If you dont it will bubble up through the molten powder. Powders vary with application you need to use an outdoor grade polyester powder which holds its colour and shine better than most of the regular indoor powders. Powder coatings are often used as an alternative to paint for finishing the external surfaces of engine blocks heads valve covers oil pans timing covers and intake manifolds. Powder coating is not just for wheels and valve covers. Its not easy to strip and refinish.
The tranny is no big deal for heat dissapation. Cast aluminum and magnesium parts may experience out gassing and or bubbling in the finished powder coating. There are hundreds of small parts parts that fit in your oven that go into a car that dont justify all the time and money using a paint gun but they are still worth more than what a can of spray paint has to offer. Both colors are from prismatic powders. I found this out on an aluminum intake. Powder coated engine parts for a customer.
But the block and head are possible considerations. Ill never powder coat engine parts again. You can coat anything that is metal. All prices are estimates only and pricing will depend on the amount of prep work involved. It just look like crap. Powdercoat will stain if fuel or oil gets on it.
One thing to take in to consideration is the more of the engine you coat the longer it will take any of those parts to dissapate heat. Requires use of high temp coating such as our ceramic coating that withstands up to 2000 degrees. It can play a huge role in your entire restoration. I used kingsport grey for the base and clear vision for the top. The most difficult items to get a smooth finish when powder coating are the ones that have been exposed to oil and grease their entire lives.