I have painted several engines with either lacquer or urethane over the years and never had a problem. The only place I ever had any discoloration due to heat was around the center exhaust ports on a small block Chevy. Preparation is the most important step. I use Gunk very liberally twice with a parts cleaning brush to remove any stubborn crud. I follow the gunk with very hot soapy water and a scrub brush. Next I blow the water away with compressed air and then spray the engine with wax & grease remover, wiping it away with a clean rag before it has a chance to evaporate. Then apply the paint with no primer. I use just enough color to cover everything good and, if it needs a clearcoat I try to use just enough to get the desired effect / gloss without laying it on too heavily.