Grinding back the bearing surface is not impossible but not easy. Hand working can visually clean up the track but it is not round.
i am pretty sure the hardening done on cheap spindles is the last step, on expensive parts the grinding is- just not beyond the hardened depth.
think about how these parts appear.
a common un55 cartridge will probably work everything else being equal- might just push the chain line out a bit- front derailleur requiring adjustment.
in the last two weeks I have found two bottom bracket spindles on bikes I bought with pitting
always a bummer.