Take some close up photos of the exposed shaft. From your picture it looks like it’s cut off but maybe there’s some weird keyway that we’re not seeing.
From this blog it looks like the hero majestic used Batavus clutch parts but maybe some other clutch too.
If the shaft is indeed cut you might want to start looking for a new engine. welding on a shaft or a clutch sounds pretty advanced and you’re going to need it to be perfectly true or the whole thing could just self destruct. Putting a different engine on is probably the best bet in my opinion. You might need to create new brackets but I think it’s a much safer option than trying to weld something onto the crank.