Back in the 50's, 60's, and 70's a lot of beautiful 98 Mausers were ruined by making sporters out of them. some ended up being fine shooting, and beautiful rifles. I'll plead as guilty as anyone on that. I still have a nice 6MM Remington varmint rifle that was built on a 98 action. Fortunately for history, it was only the action I bought, and not an entire rifle. I think between the enormous number of 91-30 Mosins made, and their rather low desirability as a custom rifle, that it's doubtful a significant number would be ruined as far as historical pieces.
You can do a lot of sporterizing to a Mosin Nagant just by changing a few parts that can be changed back. Now if it comes to shortening the barrel, drilling and taping for scope mounts, and altering the bolt, then there is no returning to the original military.
Cheapshooter's rules of gun ownership #1: NEVER SELL OR TRADE ANYTHING!