ive painted about 5 mini a series engines with normal paint and they were fine
(bearing in mind they were race engines and reaching extreme temps)
the best stuff i used was from halfords made by plasticote and was gold.
used a white primers and the finished result actually looked like gold, it was awesome.
but for a head or block on a car, any paint will do
dont forget a good few coats of laquer because brake fluid,engine oil,petrol will all remove the colour