While I'm replacing the infamous upper cover U-Gasket to repair the oil leak on my '91 300E, what else should I replace while I'm at it? Since I'll have a number of parts off/loose I may as well replace the usual suspects.

What about the belt-tensioning parts?

And....does the front crankshaft seal leak on these cars or is it pretty much always the upper-cover gasket? My formerly benign leak suddenly got a good bit worse here lately.

Thanks

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2012 E350
2006 Callaway SC560