i'm sure you can weld it but i have never done it.
I would make sure that the sprocket is lined up EXACTLY where it should be and is perfectly straight.
if it's in the wrong spot or if it's not welded on straight, it will throw the chain all the time. throwing the chain on the engine sprocket is real bad because it can and will jam up inder the cover, causing the rear wheel to lock up and probably rip out some spokes and cause you to crash big time.
i would think that as long as the crank is steel and the sprocket is steel, you can weld it on...but alignment is crutial.
i would not weld the entire thing...don't run a bead all the way around the sprocket. I would just put a few short beads on it and don't overheat the steel.
That's just my opinion anyway.