I'd try JB Weld (couldn't come up with a political reference other than stretching to Jeb Bush) since I always have some in the garage. I used JB Weld on a couple of repairs to my Diesel turbo and EGR systems and they're holding up nicely.

It might be tricky if the crack extends to the mating surface. Make sure whatever patch you use is fully cured before you tighten the valve cover bolts... which really take very little tightenting.

Sixto
95 S420
87 300SDL