Sorry, perhaps I never made myself clear:
I did not mean completely weld or fill up the hole in the shaft.
The hole will have to be sufficiently filled with weld or metal filler to hold the inner half moon (woodruff key in place)
If you heat weld, you will have to replace the oil seal.
If it was me I would use a propriety metal weld/filler to sculpture the rear half moon to the shaft.
I would also consider using extra metal weld/filler to secure the belt "sprocket" to the shaft, however removal in the future may prove extremely difficult.
You will have to make sure that the belt "sprocket" is in good shape and also when fitted that it is runnung true.
The above "botch" is fairly straight forward but should only be carried out by competent person who has an engineering understanding.
If carried out properly then you should have no further issues, however a replacement crank is the professional fix.
I have not included all the detailed procedures, but the above will give you an indication of the cheap fix.
Alex.