If the surface is bare metal,then yes,id etch prime. If your painting over a good (non lifting/flaking)surface,you can get away with just the 2K (2pac)paint. You could with enough time given,get the same shine on your engine as a set of nicely painted bodywork.
On a slighty off track subject,my good mate Robbie (
http://fumesngears.blogspot.co.uk/ ) preps his engines complete. He uses his mrs' rolling pin chopped up and inserted into the carb rubbers and exhaust ports before blasting the engine with a good power washer with a sand blasting attachment. The attachments are available for about £30-ish,and work a treat. He generally leaves engine in a bare silver,but the engines would paint up beautifully.
Just a thought.

Every silver lining has a cloud...