There used to be a Special little device for that made from stainless steel where you sanded down the sides of the fin a little to fit it. It memory serves it was made by a german company made Concrete Wave. A nifty little thing, but I don't know if it is still made or if someone else makes something similar. Just glueing the old piece back will not work.
Maybe someone else can help you?
Regarding replacement fins, I think you can fins something suitable in the Select lineup. It is not super sensitive. Any good freeride fin in the right size will do.