Baz,
I used a good one step autopaint for the engine bay but I mixed in a bit of flattening agent to make it a semigloss. The engine paint is a Duplicolor product from Autozone. Man this stuff is great! It comes out of the can like You are using a spray gun. I recommend it to anyone and It would work for the engine bay as well I'm sure. That being said... It takes seven days to fully cure. So I'm sure that it is going to be durable. As far as the color the engine bay should be the color or the car body. I painted mine black only because I have not decided what color I am going to paint her. Black goes with everything. If you have the triumph motor them it should be painted semigloss black with a silver head - If you have a MG motor then I beleive it would be Burgandy block and head.. But I may be off on that last bit.
I cant wait to drop the motor in tomorrow.
Good luck with your project!
Lee