DIY engine bay painting

The harbor freight paint gun isn't all that bad for an at-home job. Just be careful tightening the air chuck into it because the handles like to crack. Other than that it works fine for what you're talking about doing. I've sprayed with them a couple of times, I wouldn't use them for a top coat normally on the outside of the car, but they work fine as primer guns, or in your case the engine compartment, because the finish isn't nearly as critical. The biggest advice I would give you is to make sure you have a good water trap on your air compressor lines, and after you scuff the existing paint in the engine bay give everything a really good wipe down with some wax and grease remover before you paint it. There's all kinds of stuff in the engine compartment that paint doesn't like to stick to, and after 40+ years you can pretty much assume there's grease, brake fluid, and all kinds of junk everywhere even if it looks clean.