You can't weld that. It broke because it is a high stress area, then became overstressed when your belt tensioner failed. It'd cost more than a new (used) cover to have it properly welded and remachined to align the tensioner mount.

As to removing the engine from the car and removing the head and oil pan, probably not a bad idea. If it were me, I wouldn't be able to restrain myself at that point. I'd be pullin' out all the pistons, putting new rings, maybe bearings... rear seal... Sometimes I can't stop myself.

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1987 W201 190D