Find some 300 WSM, 300 Win Mag brass or ammo and see what chambers. Or, measure the opening in the receiver. Do you get <3”, >3.3” or closer to 3.5”.
I took a 30-06 (.473) bolt face and rebarreled it to a 9.3x62mm. No feed rail mods. It is a great shooter. It’s the green stocked rifle in my avatar next to my PH’s 450 Rigby.
If the action is a magnum bolt face (.532) then cartridge offerings are in magnum BF cartridges. So, let’s start here and work forward. What game are you interested in hunting with it… Plains Game (PG) or Dangerous Game (DG) or both… 1 rifle safari?
30 cal:
stay 300 WM, bullets and ammo readily available most places
300 WSM, since a long action, you could throat the barrel to shoot 3.1” OAL ammo.
300 PRC, it will have more thump on both ends than a 300WM.
300 SAUM, it bit shorter case than the WSM but you can seat longer bullets further out and get more FPS (if you need).
7mm:
7mm Mag, slightly less than a 3000WM
7 PRC
7 SAUM
416:
416 Taylor
416 Ruger
458:
458 WM, 458 LOTT
It boils down to the game hunted.