i know from experience JB weld does not work very well for patching holes in the junction area of exh.pipe to muffler. it tends to run as well as begin to bubble as if in blowing out, the JB weld patch with mesh was about 1/8" thick. Did this repair on all 4 HM100's on a K1 i had.
BUT, this area on the crankcase does not have any of the same pressures or temps. The only thing i could have concern about using JB weld is heat + time + oil affecting the JB weld, which is unlikely and can always be replaced.
I was impressed with the Alvin Products link Donzie sent.
http://www.alvinproducts.com/ The product is designed specifically for heat applications. If a non-ferrous repair is to be applied, i would use the best product i could find.
I am sure JB weld would be fine, but the Alvin product is designed for heat.
Not sure one needs to go to the trouble of welding/machining, considering what type of service condition the repair will be subjected to. The welding would be no problem. It's getting the case set up on the table of the mill to get the weld repair parallel to the table to mill off the weld that will be a major task.
I vote Alvin Products.