Sorry to hear of this problem. Whatever you do, do NOT turn the engine at all. That bolt head, if it were mine, would be resting on the timing chain and crank gear, ready to be rolled in between the two. I would pull drain plug and fish around with one of those magnets on a rod. Or remove the pan. Also, there is a tiny flash light now, LED, on the end of a rod, that will drop down into the chain cavity so you could see if that bolt head is sitting somewhere.
BTW, years ago I dropped a small bolt into an oil drainback hole in the head of a Mazda. The bolt fell into the rear of the pan. The pan had baffles and also the pan could not be removed with out pulling the engine up above a frame member. I was able to fish out the bolt with a magnet on a flexible rod, after several hours.
And, I run Fairmont motorcars (
http://motorcar.winkworth.us/ is one of my friends in this, and I have a car like the one shown) These cars are water cooled and are full of steel bolts in Al waterjackets, and Al heads. We've all had broken bolts such as yours, because the machines are usually over 50 years old. It is not too hard to get this fixed (extract and replace), but I highly recommend you find someone who has done it many times. I can do them now, and your bolt is in an easy place to get to, but I would not do it on an SL, I'm not skilled enough.
As the SLs age, that Al head and steel bolt stuff is all going to be a problem (dissimilar metals). It is really nasty when water is involved, and the bolts end up rotten stumps. UGH.
Good luck and let us know what happens.
Ron
PS There is a local welder who tacks a piece of steel onto the stump, then heats the head to get the bolts out. I would not watch that, nor do I think I'll let him do it to my car.
And, since my engine is open right now, I'm going to take those bolts out and put some "never seize" on them.