As Joe Maurer points out, original engines usually bring higher prices at auctions than restored ones - in part maybe because the purchaser has the option to restore or keep the piece original. We've all seen the disappointed faces on Antiques Roadshow when owners are told that their antiques would be worth two or three times as much if they weren't refinished. Most of our engines are real antiques! Once paint, striping and decals are sandblasted or painted, a piece of history is lost forever.
My personal preference is: no refinishing no matter what! The only exception I make is for repairs, in which case I use a rust paint, which is a blend of Rustoleum rusty metal red primer and flat black, to match the repaired area to the rest of the engine's surface. I also try to preserve ingenious modifications or repairs made to an engine - even parts made of wood - because they are part of the engine's history and make it more interesting.
When engines are refinished, I like to see parts that were not painted at the factory (flywheel rims, pushrods, pulleys, etc.) left unpainted. Those surfaces make an attractive contrast to the paint.
I realize that the choice of paint vs. patina comes down to what makes the owner happy. It's the owner's engine, and only the owner knows what will bring him or her the most satisfaction.
167 Cushing Road
Friendship, ME 04547