As mentioned above, and having personally owned and worked on a W203 for several years....I highly doubt fitting the 603 into the W203 is going to go very smoothly...I'd find a different smaller engine like a 602 or 601 if you want super ease of install/efficiency. I had a 601 w201 and loved it.

Also keep in mind the 603 is likely a good amount heavier than a 611

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