Wow! Never had one stuck like that! I wonder if someone neglected to put grease on the shaft splines when reassembling.
By beating it until the e-clip holding the pinion gear on failed, I hope nothing got damaged there...other than the clip. At a minimum you're going to need to take out the prop shaft holder and find the clip or pieces of the clip. But you would have to do that anyway to put a new clip on.
If it were me, I would spray some penetrating oil, specifically Aero Kroil, (the best I know of) or PB-Blaster (works) up in the shaft to crankshaft spline area and then WAIT for it to work it's way in there. If you have the ability to drain the oil and set the power head upside down, this would be the best scenario for the penetrant to do it's thing. If not, you might need to spray the splines multiple times since gravity is working against you here.
The penetrant "creeps" (Aero Kroil says right on the can "the oil that creeps") into rusted splines and will, eventually,, break the oxidation bond down. But you have to understand that this can take a long time. I once waited about a month to have the oil work into the splines between a drive wheel and and an axle shaft on a riding lawn tractor. I would never have separated the two without damage if I had used heat or brute force. The oil can truly work wonders if given enough time.
Most people will NOT have that much patience or grass that grows that slowly:>).
But if you can let the oil work for as long as you can stand it, the penetrate WILL help here.
Since you will be applying force against the thrust washer for the crankshaft while trying to pull the shaft, you should use a technique called PERSISTANCE rather than pounding the CR@P out of it, that would be to your advantage going forward.
Buy TWO new e-clips for the pinion. Put the pinion on the end of the shaft and you will then have a place to rest your wooden drift or a purchase area for a SMALL slide hammer claw.
When you get the shaft out and install it into the gearcase, use the SECOND e-clip you purchased to install the pinion gear and throw the clip used for pulling away.
I wish you good luck.