I use wood chisles in various sizes to scrape alot of the real deep grease off followed by drill with various size wire wheels. Next spray down with oven cleaner and let sit for a good long while. If still not to your liking repeat the oven cleaner again. Final is wire brush loose paint and grease then paint.

This



with some patience and elbow grease (not the same grease you will be cleaning off) will look like this




After this cleaning some engine paint leads to this.



Finally installed



Again be patience and use alot of elbow grease. Rustoleum engine paints used for the engine block. Valve covers are just a gold standard rustoleum paint.

Good luck with your clean up. Post up some pics of how it turns out.