Valve stem seals? Well, let the car idle for 5-10 minutes and hit the accelerator. If a cloud of blue/white smoke comes out, then you're ready for seals. Although you'd also notice oil consumption of about a quart every 200 miles. The term "missing" is kind of wide open for comment. Maybe the rough idle is due to fuel injection problems. How many miles does the car have? If it's around 100K, you might as well do a major service to it while you are doing the seals. So that includes timing chain, tensioners, and, should pocket permit, fuel injectors, distributor head, warm up-full load enrichment compensator, and the like. Ok, hope this helps!
'67 250SE Cabriolet
'93 300E 2.8