If your old board is a Sailboard Masterclass, you may be able to buy an new "euro box" or "A" box fin
and simply slide it into the rear fin slot.
How does your old broken fin mount?
If it has one small screw at the front or the back that tightens and small square plate nut down in a horizontal groove in the rear fin slot, you are in luck.
Any similar size "A" or "E" (for Euro) box fin should slip right in with perhaps a little filing and fitting on
the root of the fin to make it a nice tight slip fit in the fin box.
Can you post a photo of your broken fin and the fin slot in your board?
Do you have the mast base fitting? That may be a bigger problem than the broken fin.
Hope this helps,