Drop a 500 Euro motor in. It should bolt right in with few adjustments at all. You obviously have a Euro model already since its an '81 so it won't be much of a change. Horsepower will crank up from about 218 to about 240 or so. (I've heard horsepower quotes from 231 to 240 depending on the source for the 500 Euro motor. A 560 will fit as well but you may have to mess with the fuel delivery. 500SL Euros move pretty well.

__________________
Mike
-W126C 560SEC 040 Black
*6.0L AMG M117 32v Hammer
-W163 ML320 Brilliant Silver
-W124 300E Pearl Black