ORIGINAL: Charlie P.
I use a stainless steel brush and either acetone or oven cleaner. I'm leery about anything too "invasive" into the bearings and innards like a dunking in hot anti-freeze unless the engine is a real mess.
A wire wheel on a Dremel can make one look perky in short order, too.
I was thinking about trying oven cleaner. I have some extra 2-strokes just sitting around, and I noticed that auction shoppers seem to equate "shiny" with "good" for some reason.
I actually like having a nice bit of varnish on my engines as it shows they've been used how God intended them to be. Still, if I'm going to pass them on, I may as well try to get top dollar for them.