Yes, there are K98 barrels out there; Gawd alone knows how many have been turned into custom rifles over the years. Swap takes only half an hour if you have the tools handy and know how to use them.
I do believe that Brownell's lists 98 barrels in military profile, Douglas IIRC, which means 6-groove button-rifed: good barrels. Trouble is getting one into this Free Country of ours now that the Free Country to the South of 49 has decided that WE are The Enemy. (Personally, I think we should stop selling them petroleum products until they see the light.)
There is one other point, too. Even a pitted bore very often can be persuaded to shoot fairly well, if you go about it the right way. What you need likely is oversize bullets, but those just are not available. There is, however, another way. First, scrub it CLEAN, as suggested, then load up a box of shells with FLATBASE bullets and a fairly FAST powder (3031 is a good choice, or 4895; nothing slower than about 4064, though, definitely no 4320 or 4350 or 4831). Seat your bullets OUT a bit. WEIGH your charges individually.
Tighten your ACTION SCREWS. Single biggest cause of Mausers shooting badly is loose screws.
Now head to the range and SLOWLY put your 20 rounds through the rifle. First 5 should tell the tale.
Flatbase bullets obturate when kicked on their little butts, and a fast powder kicks them harder and faster than a slow powder, making them swell and seal the bore.
It has worked for me a couple of times.
Worth a try...... and it gets you an afternoon at the range.
Hope this helps.
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