Yank:

Before you go to the trouble, I'll note that I have a 1920s German double rifle in .250-3000, which is a rimless cartridge, with an extractor similar to that which you've described. I've put several hundred rounds (both factory and handloads) through it and never once had a failure to extract.

Unless you're planning on using the Valmet on dangerous game, I'd leave it alone, buy and shoot a bunch of .30-06 ammo through it and enjoy it.

Rem