Have you checked Craigs and eBay? That chuck looks to be a 6x12 and new ones aren't all that expensive and the machine itself looks to be for some particular purpose rather than a general purpose surface grinder. Have you shopped around? You might be able to pick up a decent surface grinder with a mag chuck to boot. I see that combo all the time.
BTW: Some time ago, I got a couple old (and cheap) mag chucks off eBay that were pretty beat up. They were covered with serious "divots of shame". The oil in one of them was so sludged-up and filled with swarf, it wouldn't operate. Anyhow, after completely disassembling them, making new handles, installing new seals and oil, surface grinding the tops and bottoms... they work perfectly, smooth as silk. The 6x18 is true corner to corner within +/- 0.0002 and 6x12 is dead-on to the best I can measure. There were a few divots too deep to go after but, nothing to worry about. I'd say they're in 95% good condition now.