Old school repair guys used a lot of different stuff that's no longer available due to being... Not good. Ha. But naphta was one that's still widely available as white gas aka "coleman/camping fuel" Essentially naphtha with stabilizers to keep it from going bad. Mineral spirits works as well, but naphtha leaves less residue in my experience. Lacquer thinner is good for especially stubborn crap and cleaning slugs, but evaporates really fast and the fumes are no bueno. Alternatively; non toxic degreasing cleaners like simple green are usually my preferred method of cleaning up especially gross machines. Typically very safe on paint finishes and internals, just make sure to keep it off the decals. (It can and WILL erase them if it sits for more than 10 secs) Really though, nothing beats air and a long handled "paint" brush. My air compressor and blow out tube are some of my most cherished tools.

Ultra One, a restoration firm whose specialty product is a de-rusting solution which is designed to remove only rust while leaving the underlying metal, paint and decals intact.