It really depends on the board. I am putting this simple, because I am no electrical engineer. But anyway, the board might have a setting for higher frequency CPUs or you might have to replace the crystals. They usually came in 8 pin DIL package and there many times is a socketed spot on the board for this.
Amiga kit sells them as do many other electronics supply places, like Digi-Key, Jameco and mouser.
IIRC, you had to match the crystal with the CPU. So if it is a 50mhz CPU you had to make sure your crystal was also 50mhz. Again, I may be wrong but this is what I remember doing back in the era.