Date registered: Aug 2002
Vehicle: '85 2.3-16 '99 C280 '11 GLK350
Location: Scottsdale, Arizona, USA
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Your engine swap potential would be better with a six, since it makes the M104 engine available, since it is wire harness compatible with the M103 six, with the added benefit that there's an AMG variant. To fit a six in the four cylinder engine bay would require changing the front radiator setup and quite a bit of rewiring.
The only reasonable transplant upgrade for the four cylinder M102 is the 2.3-16 engine.