Not the worst I have ever seen......like half the spline width worn away. Looks pretty good.
In the short term, my cheapskate method is to use the stainless steel support strips taken from an old windscreen wiper blade, cut down to width and length, coated with Loctite and hammered into the gap between the crank splines and the drive sleeve inner splines. It may take a couple of strips to fill the wear gap on each spline. Good material to work with, well up to the job.
Requires a bit of trial and error, but will save a crank from an otherwise expensive repair. Warm it up with a hot air gun if it ever needs to come apart. Obviously treat the motor with a bit of respect, so as not to apply sudden, heavy loads.
The wear is caused by the big nut on the crank not being tight enough, allowing the drive sleeve to fret on the splines. When assembled correctly, two or three threads on the crank end should be within the end of the drive sleeve. This allows the big nut to bottom against the drive sleeve and push this down the crank to lock the sleeve and main bearing inner race against the crank cheek. The problem with a wear pattern like this is that the drive sleeve may snag on the worn spline, so must be rotated away from the worn crank spline side to make sure it can move down the crank splines as far as possible. Do the trial assembly without the cush drive.
You could also have worn or missing circular crank shims and wear to the crank cheek bearing inner race location, but from the picture this area looks reasonable. There is also a witness mark where the crank cheek has rubbed against the main bearing outer race, indicating loss of crank shims or incorrect initial set up, allowing the crank to move sideways and make contact. Check for the correct location of this outer race, it should be a tight fit in the crankcase.
Best to get some new sludge trap plugs, yours look a bit butchered. They should not be screwed in more than the crank web face, to avoid blocking the oilway from timing side journal. Some pattern part plugs are made too long, and need shortening. Make sure that trap is nice and clean. The design of the oilway and sludge trap differs depending on the crank. Early small journal or later big journal.
The delights of shimming the crank end float await. When the float is correct, chain alignment sorted, add your strips just before the final tighten.
Lots of information on this forum about the big nut, and its high torque requirement. Plus how to shim the crank end float.
Swarfy.
