Looked at the J-B website (http://jbweld.net/products/jbweld.php). From
the specs on the page, I would guess that repairing a cracked carburetor
would work, but the result would be weaker than the original. Probably
strong enough if it is protected from being hit or as long as the tiller
doesn't fall on it.

If it were my tiller, I would try it. As I recall, the OP had borrowed
the tiller. In that case I would go for a replacement carburetor to be
able to return the tiller in the original condition. (The carburetor
would be chalked up to educational expenses).