Back to the question, you can clock it as low as you want and it should work (slowly); over clocking (let's say to 36MHz) may be too fast for the CPU switches or the refresh rate of the ram. I had an A2630 that choked at 28MHz, and a GVP 3001 that froze above 32MHz.
Look online to the X-bays, Jameco, and DigiKey for a variety of crystals; and remember that the first true digital watch used a tuning-fork -- bloody 1960's!
