Good News, that's a pretty good pump. Berkeley is definitely one of the preferred pumps in terms of parts availability and performance potential. The model 12JB should have a split bowl, which is a good thing. This means that you can use bolt-on accessories such as drop snoots or wedges in order to tune the ride and attitude of the boat. The -B extension indicates that the pump is (or was originally) equipped with a B cut impeller, which is well suited to use with a small block engine. The big block engines generally came with A cut impellers. The B impeller will allow the smaller engine to turn more rpm's, allowing it to get closer to it's power peak. There is a wealth of information available here, and at some other jet boat related sites on the net.
SteveM.