You have my sympathy.
There is not much free space in a bell housing. One little nut getting caught by the spinning flywheel/clutch is likely to break out through the casing. It MUST be retrieved, whatever the work involved. Fishing with a magnet on a string may work, but unlikely. It may have dropped to the bottom, or be sitting on the clutch somewhere. It is small enough to get into the clutch cover and jam its movement.
If there's no hole or cover underneath, it will have to be a gearbox out.
Are you sure it fell in there? Not sitting on top of the gearbox, or caught somewhere. Rolled into the grass off the edge of the drive, sitting in the corner of the garage, etc?
Good luck.
The little hole in the top used to be for the timing marks, they originally had a rubber plug in them. Once the problem is sorted, some masking tape might be a good idea to cover it.