The Finnish rifles were mostly reworked Russian ones anyway. Finland reworked Russian/Soviet rifles in part because they never bothered to set up a plant to make the receiver, just made all the other parts as required: good idea for a small country with a big enemy. Finland only became industrialised to any extent after the Second War.
As has been pointed out already (and thanks to Claven2) you can keep the headspacing perfect by replacing the entire bolt EXCEPT for the bolt-head. You keep your own bolt-head because this is what the rifle headspaces on, same as a Berthier, a Dutch/Romanian Mannlicher or a Ross. The LE has its headspace ADJUSTED by switching boltheads but the headspace itself is governed by the fit of the bolt body into the receiver (which the British call the BODY). Each one has its own little tricks.
Good idea is to inspect your bolt-head carefully for flaws. I picked up a Tul'ski 1906 at a gun show, discovered that it was shedding a lug when I got it home: lug was ready to separate from the head, big, nasty crack. New bolt-head cured that.
With rimmed cartridges, headspacing is nothing like as critical as with rimless ammunition. With rimless ammo, you have to be sure the round does not go too far FORWARD in the chamber, thus producing a potentially-disastrous situation. With rimmed ammo, the RIM prevents the case from going too far into the chamber. Generally, if a rimmed round is held to the bolt-face by the extractor, you're safe enough... granted that your ammo is up to snuff. Sure would not want to try it with 75-year-old wartime brass cases, though: they have had much too long to age-harden, as brass does do.
The Finn Model 39 generally is accurate enough that it really should be a handloading proposition. Surplus ammo, if you can get it cheap, should be regarded as a source of components. It is rather surprising just how much you can impove the shooting of many old rifles, just by carefully unloading surplus ammunition and reloading the components into fresh brass with new primers. Modern primers have essentially an eternal shelf-life, especially if the shellac sealant is undisturbed. When a primer is loaded into a casing, this shellac seal is cracked and solvent outgassing from the propellant can affect the priming compound adversely. So you unload the old ammo, throw new primers into the equation.... and the old components then tell you just how good that ammo was when it was made....... and most of it was pretty darned good.
As to screw-holes, the milsup gods have given us plug-screws and red Loctite, Birchwood Casey Perma Blue and very tiny needle files. The screw-holes can be plugged off and hidden...... or they can be kept and the rifle re-mounted with a scope some time in the far distant future. Perhaps they detract from the "value" somewhat and I can understand that but, given the choice between a pristine clunker with a grody bore, and a D&T victim that shoots like a charm....... I would gravitate toward the D&T victim, at least if I had any intention of shooting the thing. A frend of mine D&T'd a whole collection of prime milsurps over the years because he was TESTING them.... and some were good ENOUGH that they NEEDED those 16x and 24x monster scopes he was putting on them. I do not argue with 1-inch groups at 300 yards, year after year, from the same Norski Mauser. Some of them are GOOD.
Hope some of this helps.
Nice rifle.
See you at the range, I hope.
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