Multiple Steps
First, blast away everything you can with compressed air. This is where one of the long, narrow tubed air guns comes in handy.
Second, soak what can be submerged in a solvent, Purple Power or Simple Green work well. Use an old toothbrush to scrub the parts clean. Repeat the first step.
Third, get out the carb and choke cleaner and some dental picks to finish off the really caked and nasty crud. Repeat first step.
Fourth, use some quick detailer, I like Meguires. It will impart a nice sheen to the plastic and painted parts.
Fifth, grab a fine point black Sharpie and recolor in all the Stihl logos in the castings that were originally black.
Optional step that I've always found worthwhile, especially if the saw cleans up nice, is to get a cheap new bar and throw it on with a new chain. Keep the old B & C to install for test run purposes.
Take Care