The shifter takes a lot of abuse, that said, it will be some decent steel and it will not be easy to cut and weld. Take that back, it will not be easy to weld it and be confident it will hold and last.
From looking at a picture, I say it is a forged piece. Myself I would measure splines and tried to find an adjustable shifter from another motorcycle. Did you check with cyclex and/or DCC? I would expect racers dealing with similar issues.