You need to remove the remnant of the old key- it is milled in sync with the crank throw and if you change it you will be horribly out of balance.
The sheared key is not welded to the shaft- it just looks that way because the flange for the blade spun over it. If anything would have friction welded it would have been the flange to the crank.
Take a file and lightly file the area where the keyway is- you should be able to file away the spun garbage from the flange off and reveal the key.
When you do take a good quality punch and you should be able to dislodge the key. If you have some penetrating oil that will help.
If that doesn’t work the the old key would have to be milled out.