Conversion to .30-06 was done to quite a few MNs, decades back when they first came on the surplus market. The common advice for these guns today is -- don't fire them. The consensus seems to be among sources I have read that the pressure difference between the 7.62x54r and the .30-06 is too great to be comfortable with in this action.
Is that true? Heck if I know, but i am sure of something -- the 7.62x54r is nothing to sneeze at, and comes darned close to being as useful as the .30-06, and the action works like a charm when chambered for it.
IMO, the MN is as perfectly what it was designed to be as it can be, just as it is. As-is, they are darned nice rifles in many ways.