No reason why those hole can't be welded shut and the bore relined with a smaller bore rifled liner. I'm not sure that you could safely reline it to 8x57R, which can be a very potent cartridge, but a milder, but still big game-capable rimmed cartridge should be possible. How about .30-30? Or 7-30 Waters? Or 6.5x57R? Not cheap, but doable, at least here in the US.

I leave it to the "metalurgically literate" to decide if you weld first and line later, or vice versa.