Last owner apparently not a big fan of changing the oil

I recently bought a 1973 Duster with a /6 I eventually want to swap the engine but for now the current engine will have to do. I took off the valve cover and a good amount of the gasket was missing. And there is quite a bit of sludge. Baisically I have two questions. 1 whenever I would accelerate there was a sucking sound. Could this have been caused by the parts of the gasket missing? And 2 because of the sludge would you guys just suggest cleaning the cover,rockers, push rods ect. Or should I further tear down the engine? If so how much? Proboally some stupod questions but im still rather new to engine work. Thanks for any help!1484512230532-275128269.jpg1484512286890222968839.jpg1484512230532-275128269.jpg