8x58R sauer cases are not that hard to get so you can load yourself or have it loaded by a custom loader.
If you can get a gunsmith to rechamber to 8x57JRS be aware that this powerful cartridge operates at much higher pressures and the conversion would certainly require reproof.
Old drillings with seemingly allmost no rifling left can sometimes surprise us so presume nothing and rather do a test firing (before any conversion).
With kind regards,
Jani