There were only 17 million Russian Mosin Nagants made, so they are not as rare and collectible as a Finnish Nagant.
I have a 91/30 with matching serial numbers, receiver marked 1926, so that one I am keeping pristine as possible (although I suspect the importer gave the receiver a flat black refinish job).
On the other hand, I have a Mosin-Nagant carbine with different serial numbers for receiver, bolt body, magazine floor, and butt plate. Headspace is OK so THAT one is fair game for tinkering (no historical value).
But there have been two rifles I owned, Spanish 1916 Mauser in .308 and a 91/38 Carcano, that I wish I had resisted the temptation to "sporterize" but those of us in the 1950s and 1960s were brainwashed to think "sporting purposes" not "curio and relic."