Last time I did this, I tied a thread around the original bead and tied/taped the other at the breach (it's a single shot hammerless). I marked the bead location before making the cut in the barrel.
After making triple-sure gun is unloaded (you could remove trigger group as a precaution), you could put a thread down the top of your barrel and receiver with the ends taped with masking tape. Now you can lightly clamp the gun in a padded vise and look at it carefully from both ends. The human eye is good at centering things, especially if you can step back a bit.
Mark your bead location and check again to see if it looks centered from both ends, from above, etc. I got drill bit and tap as a set from Midway, IIRC. I bought the replacement bead at the same time. I don't suggest re-using the bead. You don't know what its threading is, and it's hard to discern with such a small object.
Make sure to clean up the hole after drilling and after tapping, being very careful not to mess up the threads. Check to make sure your bead's shank doesn't protrude into the barrel. I'd use a file to shorten it, if needed, and I use a tiny drop of blue Loctite on shotgun bead threads.
All my best,
Dirty Bob
