Rough idling and shaking from engine '87 450SEL

This weekend my mother was driving a long distance in her '86 Mercedes

450SEL and it started to run rough. When she pulled over there was a tiny bit of smoke wafting from under the air filter which sits on top of the engine. The smoke stopped once the car cooled down, but the engine still shakes and idles more roughly than it used to. There is a noticeable clicking/ticking noise than gets louder and faster as the car approaches 2k rpm. There is a slight hesitation in acceleration. The oil pressure and temperature are consistent and normal. There are 194k on the original engine.