Lots of ways to go. I'd likely use Oven-Cleaner, Degreaser, soak in water/ laundry-detergent, scrub with bristle-brush and steel-wool scouring pads, etc. You can remove old paint with "Aircraft" Paint Remover.

An engine-rebuild shop could either hot-tank (if still available) or use their more environmentally sensitive alternative cleaning systems.

Another choice might be Media (Sand) blasting followed by washing in soapy-water to remove any residual grit, ... lots of choices.