Painting a K161 question.
 

After I de-grease and clean rusty spots, do I have to prime this engine, or would it be OK to just paint over old paint?

Don't see much for paint. When you remove the rust you'll need to prep the bare metal before painting, so I would suggest priming first.:beerchug:

if you don't prime it the paint will look horrible , I always primer my parts even if its got good paint under it, need that clean surface to show nice paint. Best thing to do for engine parts and small items is to use a sand blast cabinet, works really nice but you need a big air compressor. you can hand sand most parts as well just get the bulk of it away so the new paint will stick.