If you swap a LOT of mechanical parts, yes, it can be done. Have to move all the distributor ignition setup (and valve covers, and cam sprockets) to the W210 engine. You'd also need to get a 4.2L LH module and EZL for the R129.
If you are based in USA, just get another SL500 engine instead - they aren't that expensive. And you won't devalue the R129 in the process.