The threads probably got damaged when the old bolt loosened and fell out.
The bolt threads into a pin that is tapered on one end. The tapered end fits into a tapered boss that is welded into the end of the lift arm. Normally when the pin and boss are in good condition, they will fit tightly and grip as soon as the bolt draws them together.
If the pin was loose for any amount of time, the taper on the pin, and the taper on the boss are probably also damaged.
To remove the Bob-Tach, you loosen the bolt and smack it with a large hammer to release the taper pin from the boss. You then unscrew the bolt and use it to push the pin into the Bob-Tach. The hole in the Bob-Tach is deep enough to accept the entire length of the pin.
I would suggest you remove the bucket, then drive both pins into the Bob-Tach and pull it loose to clean an inspect it. You may be able to re-tap the threads in the pin. You also need to inspect the hole in a tapered boss. If the damage is minimal, you may be able to clean up the tapers enough to get them to seat an lock up.
If you want to try the quick and dirty method, you could try to thread in a bolt and use a slide hammer to try to seat the pin. I would highly suggest you take it apart and inspect it. The cost of replacement pins isn't too bad. A damaged boss can be reamed if the damage isn't too bad. However the reamer from Bobcat is $$$.
MaxW and OldMachinist have determined that the reamer is the same taper as a ball joints and tie rod ends. You can read the thread
Pivot Pins 443B
There are numerous threads here on this repair.