not sure about inlet manifold but rocker cover can be easily cleaned with a good degreaser like bilt hamber surfex HD, i'd say clean the inlet manifold in the same way, but then follow the manifold off with some metal polish.
Assuming they are aluminium, Jizer first, followed by any good wheel cleaner such as Wonder wheels. It's unlikely you will get the just cast finish back without beadblasting or vapour blasting, but you may get close.
