I am nor aware of an 80 hp jet boat (by mercury).
I had a 93 with the 90 HP motor and it had the 90 hp three cylinder.
I had bought it for the parts especially the crank, but it had a different spline than the 95 mercury I had in my Hobie Jet.
The outboard cranks are not alleged to fit the jets. I cannot prove that is the case, but I do know that there is a difference in the 93 and later.
The outboard has a different baseplate to bolt to the jet pump, but these can be swapped between the outboard and the jet motors.
If there is a difference it is not in block or crankcase, but rather in the splines in the crank.
Removed - Mod Team
You could check the part numbers of the cranks between the 94 and 95 jet cranks and the outboards.
I had thought that all of the jet motors had the same splines up until I rebuilt a 93 motor (90 hp) and tried to mount it on the pump.
I ended up taking it apart (I had had one cylinder sleeved) and replacing the earlier crank with the one out of the old motor.