I patched mine with quick set aluminium putty after a low side, to complete the last leg to Phillip Island then back to Canberra. It was what a friendly local had on hand and worked well enough, with only a slight weep. The hardest part was getting it to stick to the alloy rather than my fingers - wasn’t pretty.
JB Weld should work fine for a crack provided you degrease the surfaces properly. Apply from the outside rather than inside to avoid the potential for bits to flake off and find themselves floating around in the engine.