A catalyzed urethane paint - common stuff - is even resistant to lacquer thinner. If it's a metallic color, I'd go with a 2-stage (basecoat/clearcoat), easier to "save" in case of application problems. I like Standox as it's high-solids & covers in fewer coats than other systems I've used. They also have different primers, including one called Non-Stop, that will cover anything and lock it down.
If you've gone down to bare metal, a coating of etch primer goes down first.
The smaller guns, aka "touch-up guns", are more maneuverable in an engine compartment, but I've done well with an old suction gun. But it's days are numbered, gravity feed's just so much better.
Engine compartments require a lot of gun-trigger modulation and planning the shoot a little: shock-tower backsides 1st,
then firewall (some spraying upwards there), then aprons & backside of rad support, then exit through the bottom.
Take your time and practice on something first.