I recently bought a 1985 190e. I was told that it had just had a valve job, and it looked like the head had been hot tanked. Within a few hundred miles there was some serious valve noise so I pulled the v cover, and this is what I found: all eight of the tappet contact seats are missing AND the cam gear is almost gone, literally ground away from the back forward by the chain. Since this is the first MB I've worked on, I went out and got a manual. From the photos in it, the cam gear seems to be located on the cam correctly. All the rocker arms are nicely aligned with the cam lobes. The chain and all but the cam gear are aligned straight. The cam gear is about a quarter of an inch too far back to be aligned with everything else that the chain goes through, over, on, etc.

The tappet noise is a no brainer, it’s coming from the absence of the tappet contact seats. (The top of each valve stem now has a worn, cup shaped area at the valve stem end, thank you norm's auto repair, San Jose, CA).
However, the cam gear is another can-o-worms. I can't see any way to realign the cam gear with the rest of the gears, guides, and chain. What would cause this? How to fix the situation?