I found a nice tool, a bendable stick with a magnetic tip at the end. Right now I have been fishing, but haven't had any luck.
I drained the oil and looked at the oil pan. No nut. It did however contain one or two small metal and a few tiny plastic bits, and the oil filter had some woolish material in it. The wool probably is a mix of steel wool, gaskets, filter material. The plastic would probably be the camchain holder. The metal bits could be everything...
I guess there is a chance that something in the top end got whacked? Or would you say that the chances of a deadly ricqochet are next to none, McRider? I'll try to pry open the top end, and see if I can locate the nut. Thanks for the tips guys, I'll let you know what happens.