As Nos says, don't start hitting it, you will break the cast plate.
I seem to remember a bolt being hidden under the trim tab on my old Johnson. might be worth a look.
It could be however that the splines are stuck in the crankshaft. The only way I ever resolved this problem was on an old 35HP Chrysler.
I got the engine good and hot before trying to remove the bottom end, but on that engine I was also able to point a cold hose directly at the shaft through an exhaust port. Cooling the shaft and heating the crank sorted it.
It could also be corrosion around the stainless dowels between the castings, which can sometimes be loosened off with a gentle tapping on the side of the casting, but I stress gentle tapping with a soft mallet.
Bring it up the church Monday night and we'll all have a go