I’d leave it alone for several reasons.
The value is what someone will pay for it, which could be significant based on condition and maker. Date of manufacture used to matter, with rifles made after 1943 being less valuable due to quality issues, but that has become less important as people have begun to collect them more for their historical value than as a basis for a sporter.
To assess value, you need to identify the manufacturer. The key to this is the Waffenamt, a mix of letters, numbers, and other characters stamped on the receiver ring. The Germans used it to evade the limitations imposed by the Treaty of Versailles by concealing who was building military equipment for them. From there, online charts can tell you who made your rifle and you can check gunbroker.com and other sites to see what they bring.
Lots of different factories made rifles. Some made a lot, others only a few. In decent shape, a working rifle from a common manufacturer with correct (but not matching) parts can fetch $6-700. A gun with matching parts could bring double that if it’s not a fake. Sky’s the limit on value for the truly scarce versions or anything in really good condition.
Serial numbers can be a red herring. Manufacturers often numbered rifles from 0001-9999, then started over, so they’re mostly useful for telling whether the parts match. Even then it’s easy to fake numbers on minor parts. Also, manufacturers didn’t all number the same parts, and a lot of them stopped numbering parts (other than the receiver) later in the war. They weren’t thinking about tracing the history of a specific rifle decades later, they just needed to keep track of them until they were delivered to the military.
Conversions range from the sublime to the ridiculous; online auction sites have examples of both. Today it costs less to buy a modern rifle than to alter a Mauser to use a scope, much less upgrade the stock, trigger, and other parts. The modern rifle will also fire a modern cartridge, likely be more accurate, etc., while the Mauser will always be an old military rifle. Be particularly wary of converting them to use belted magnum cartridges. This requires altering the feed lips and the follower. The feed lips are built into the receiver and are critical to reliable feeding. Almost everyone who knows how to alter them properly is dead. If you do anything to it, it makes a lot of sense to leave it as a 7.92x57.
Speaking of which, the cartridge is no slouch. You can handload it to match the 308 or 30-06, but the bore diameter probably makes the 338 Federal a better comparison. American factory ammo is loaded about like the 300 Savage if not the 30-30. European ammo is loaded to the cartridge’s full potential.
Finally, these rifles can be remarkably accurate, but you’ll be hindered by the military sights.
Okie John