Before you get out the old soldering iron, you do know that most CPU's have absolutely no difference between speed ratings in terms of design.
They come out of the fab and each chip has slightly different wafer processing characteristics. Chips are hand picked for stability better speeds, based not on their design, but their chemical / physical ability etc.
If there are not enough of lower speed rated parts, they'll just use higher speed rated parts and brand them appropriately.
You may have a CPU capable of 50MHz soldered down already.
Instead of swapping to a different CPU, remove the FPU, add a heatsink & Fan and just change the Crystal Oscillator (the square thing on the bottom RHS) to a 50MHz part this will make your 33MHz chip run at 50MHz
