Other than restoring something fairly far gone, I don't really see a good reason for messing with "historic" firearms. These days there are so many modern options for anything, and these options will often be cheaper and better anyways so why bother?
For example, why sporterize a milsurp rifle when you can just buy a commercial sporter for less, without really giving anything up?
Or the other example that's come come up already are the bubba'd SKS's. People can spend so much essentially trying to convert an SKS into an AK (which they can't ever fully achieve), that they may as well just buy an AK anyways.
If you have a bunch of random parts of a historic firearm, it's fine to build them into something special to you. Had a talk about this in the gun shop the other day. A guy built a rifle around a 1917 enfield action, I was shocked until he said he just had a bunch laying around with no home to keep them living.
It's important to keep old guns alive and shooting, but also in good condition.
I wouldn't take an original milsurp gun and modify it, but I would replace broken parts. Restoration is fine on guns that have "seen better days" and refinishing metal or wood is fine so long as it would not be protected from the elements otherwise.
Keep your guns in good condition, bottom line.
Yeah, these two say it better than I could. I'll still stand by messing with a mostly complete historic gun is wrong.