Spray it down with lubricant solvent. Maybe even overnight. But not necessarily so. Your aim is for it get it under the key and keyway.

Using some finnesse(key can break) hammer tap the key into the sprocket almost nothing...this is mainly just a tap really(to loosen but inward toward sprocket). Then come around the top with a long ratchet extension.

Place the female end of the extension on the top of the key making sure it is flush with the crank sprocket at same time......flip the hammer to it's side(to help not miss and take some brunt off the extension) and give a low-medium strength wack.

Grab a pair of pliers and pull the key right out.....now slide the sprocket off.
As for the crush sleeve after it you can also remove it without damaging using some left to right motions and "walk" it off. Make sure to note it's orientation.

Now access the blasted crankshaft seal and try not to mangle it....as u can reuse it.....sandwiched between a short extension to hammer the new one back in. Dont forget to lube the chamfer'd edges of the new seal.

Contrary to popular belief there is no way a puller can be used here as there is NOT enuff space to even remotely hook the puller's arms to even the very edges of this sprocket which sits in front of a metal crush sleeve of sorts ALSO preventing you from hooking a puller to pull.

If removing the key is unsuccesful I would recommend a long chisel and very strong blows to crack the sprocket off...rather than to waste time with a puller. (it will crack)