Just checked up on this and the GVP A530 also came in a
50Mhz flavour so I would think the controller is made to handle a 50Mhz oscillator just fine (if I'm not mistaking the RAM was pritty good on those cards and ran at 50 or 60ns). As your CPU is also made to run at 50Mhz it should be safe to try run it at 50Mhz. Just desolder the original oscillator and add a "oscillator holder" and try it at 50 or even 55Mhz... if it fails it's easy to change back to the original oscillator but I belive it should be just fine.
However I do belive a coolingplate of somesort, or better yet a fan on the side, would add to the CPU lifespan and happiness at higher speeds since the Harddrive is mounted right on top and will most likely add heat around the CPU.
