Is the brown disc part number 103-158-00-88 ?
The area of the engine you're in is best covered on the 126 CD-Rom in engine 103 service task 01-212 - Removing, installing top front cover.
You mention the disc being destroyed, then further down - missing. Which is it? I've been here once and can't say for sure, but it seems like you might be taking in oil if the part number I mentioned above matches the one you just bought and it's either cracked or missing.
The photo's in the service task show the disc(guard cover) affixed with a bolt going thru a triangulated piece called a driver. When reinstalling, the groove in the driver must engage the pin in the camshaft. Bolt is torqued at around 15 ft. lbs. approx.
1991 300-SEL - Model 126
M103 - SOHC
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