A: Of course you can! Once you are aware that an heirloom ring exists, make sure to tell your future fiancÚ so he doesn't go out and buy you something new.
Regardless of how old the ring is, you should take it to a reputable jeweler and have him or her check that the setting is still strong enough to hold the stone(s) in place. You want to make sure they're not loose.
If the jeweler suggests that you replace the setting, you can decide if you want to recreate the exact setting of the original ring, or you can take the stone(s) and have them set in a new setting. Many brides opt to take an antique ring and have it set in a more modern-looking setting that fits their own personal style.