OLD bottle of Testors GOLD Paint?

Since its just an engine block, just paint it on with a brush. You should get good coverage over the bare plastic.

If you were going to try to shine it up like on a set of valve covers then you'd want to apply it over a white or black base with your airbrush.

Actually, I find that "light" metallics such as silver, gold and aluminium look better on a dark base. As you may be aware, metallics have a transparent/translucent carrier to allow the metallic particles to show through, and the particular colour is imparted by tinting the carrier. 

All things being equal, it shouldn't matter what the underlying colour is if you apply sufficient thickness of the top coat to achieve complete coverage. However, if there is any "show through", the light metallic particles contrast far better against black and it often appears brighter and more lustrous,  than it does against white.

But anyway, that's just my opinion - you may wish to do a comparison and see which you prefer.

Bronto,

Thanks for confirming!

Gordon,

I have the engine block and a few other engine pieces primed in white primer.

If the piece you are going to paint is small I would brush it on or not spray it. Either way I recommend using a primer cost of white on the piece unless it is made of white plastic. Gold does not cover dark plastic pieces very well.

Yes, it is an enamel paint.   Testors thinner should work fine with it, as would any paint thinner/mineral spirits.