Here are a few things that I have done or seen done. I'll not waste the time or effort on the obvious things like cylinder boring.

Glyptal: Glyptal is a paint intended for electric motors. The paint has excellent oil and heat resistance. I used it to cover the rough cast iron surfaces inside the block, head, and timing chain cover. applying the paint to these sufaces helps to smooth out the otherwise rough surfaces that slow the return of oil to the pan. There are other paints commonly used, but Glyptal seems better suited for this purpose. This idea was borrowed from a book by "Smokey Unick".