Just what the guys have said plus added comments: No, if the ONLY damage is the gear on the flywheel/magneto rotor, that does not make an engine swap necessary.
However, there is usually other damage done when those starter/gear/clutch things go wrong in the older model Savages. And you MUST use the right kind of puller to pull that rotor, or you WILL damage internal engine parts. All added up; sometimes it is a better play to get a late model (95+) engine along with it's CDI ignition unit and replace it all. Just depends on how much money/time and tools you want to throw at it.
May be cheaper/easier to part out the old bike or sell as-is, and buy a later model one with low mileage. There is no shortage of people interested in parts, and also later model Savage/S-40 aren't that hard to find (although the problem there is, they're usually not in your neighborhood).
Unless you have an attachment to the bike because of it being "dad's" it's kind of a toss-up. I know, I've been there... decisions, decisions...