A. The 1st part of
your question; Yes, you can re-use the bearings
if they are good, and if you don't loose any. You can just clean and re-grease the whole lot and
try to get them back in there again. If the steering on your bike has a spot
where it seems to stick, chances are you have a spot on the bearing race that's
worn, or possibly a bad ball bearing or two. At that point I highly recommend
replacing them with tapered roller bearings.
The second part of your question; Here's a link to a page for Pro-Flo Bearings that was posted on the Yamaha Virago Mailing List that shows the tapered bearings that will fit all Yamaha steering heads, including the '81 to '83 models that use loose ball bearings. Converting to the tapered bearings is a good idea if/when you're steering head needs new ones.
Here's a list of links that are related to those above that you may also find useful:
Pro-Flo 'All Balls Racing Products" steering stem bearing cross reference
Pro-Flo 'Parts Plus" steering stem bearing cross reference chart