I don't think that is the best solution.JB weld is great for on the track fixes but long time issues it will fail. The vibration and heat that the cases produces will weaken that in time. Look on ebay for crank cases for your mill. Or might as well upgrade to a new motor!
The sad thing is that the motor probably ate dirt. That can shave gallons of the life of the engine.
If it's old anyway, I'd try the jbweld. Use the dremel a little on the crack to make a little canyon for the JB. This will also scuff up the surface and give the JB tooth. After it hardens, come back and make sure it's not hitting any moving parts and if so dremel that down too.