Rebarrel. You might even find an original replacement barrel if you look hard enough.
The Savage 99 is a classic and as long as the action is good its well worth restoring.
Not sure but most spool type magazines only work with the original cartridge. If you cange chambering you may have to find a spool to fit the new cartridge.
You may be able to test to be sure the rounds you want will feed properly.
Light cast bullet loads may be best for this old barrel.