Last owner apparently not a big fan of changing the oil

Actually it doesn't look all that bad - for 30 years ago before the prevalence of modern oils. But now with modern oils, you're right, it should look cleaner inside. If it is running half way decent, I wouldn't tear into any more than I needed to. Try some detergent oil additive that you add to your oil to help dissolve the crud. You add it to your oil, run the engine to warm it up, then drain the oil and crud out. Afterwards, just use some high quality oil and it will probably be fine.

The valve cover gasket isn't the source of the "sucking" sound, a major oil leak yes, but not a "sucking" sound. Depending on the type of air cleaner sitting on top of the carb, it could just be the un-silenced noise of the air being drawn into the carburetor. If it is not that, it has to be a vacuum leak in the induction system and if that leak is so big that it is making a "sucking" sound, the engine is probably running like crap.