Provide some additional details about your 1991 300E ... mileage, maintenance history, how many owners. The lower half of the engine should be good for 250,000 to 300,000 miles ... so it's unlikely that you need crankshaft bearings. When were the ignition wires, distributor cap and rotor replaced last? A new coil may improve engine performance. I'd try the inexpensive maintenance items before doing a major engine rebuild.

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Fred Hoelzle