Thanks relative4. This is very valuable information.
BORESCOPE:
I will get a borescope with small camera and put it down the hole, see if I can find the piece. But I will probably not find it this way because, as you say, the broken wrench fragment likely ended up in the crankcase. But seems worth trying the borescope first.
PULLING THE OIL PAN:
I looked at the oil pan and unfortunately removing the oil pan on that car is quite the job. The intermediate axle goes through the oil pan (see parts diagram). So reading the shop manual I’d have to remove ball joints and axles plus the differential. Worse, shop manual says I’d also have to remove three engine mounts which means I’d have to support the engine from the top with a hoist since I cannot put a floor jack under the pan. I don’t have en engine hoist. Or is there some other trick?
LEAVING THE BROKEN WRENCH FRAGMENT IN THE ENGINE:
Here is something interesting…
Looking at the diagram below, if the broken wrench fragment made it into the crankcase, then it is likely sitting on top of that baffle plate marked #5 in the parts diagram. If the fragment is not sitting on the baffle plate yet then perhaps it will soon make its way there — as soon as there is some engine movement. I will first turn the engine by hand a few times if I decide to go that route.
Now if/when the fragment makes it all the way to the bottom of the oil pan…
…then I might have a chance to get it out through the oil drain hole. The fragment is only 6.5mm wide and the oil drain is a 12mm bolt, so the fragment should be able to pass through the oil drain.
The oil pan is aluminum so if the fragment ends up at the bottom then using a strong magnet and dragging it on the bottom of the oil pan I might be able to direct the fragment towards the oil drain opening.
Once I have the borescope I can also put the camera in the oil drain and may be able to see the fragment, in which case I may be able to pick it up with a small magnet attached to the camera.
So, are you reasonably sure that this hole leads to the crankcase? (I guess once I get the borescope camera and put it down the hole I may be able to see where it ends up myself)