Its not about bolting on parts, these old w210 are not fully obd compliant and still require a muxer to interrogate the units from the 38 pin connector.
Thats the start of the story, then comes understanding the stupidly coded errors mercedes used in them, some required a simple math subtraction to get to the original code. Lol (the deeper codes are do not start with U (and mosy euro market cars didnt even have a 16 pin connector to show P error codes like on US market mandate.
And when one unit errors out, it locks. Then it requires commanding to unlock, unlike later models which self reset sometimes.
TLDR, its possible to shove a petrol engine into it provided the person doing the job has a mercedes tool and knows how to use it and also has tools to code the ecu.
For love of 2jz, w210 also came with m104 (3.2 straight six) in 1997. But those cars didnt have electronic key like this one, the M104 did come in the very last w140 S320 which had the electronic key. The coder needs to be smart to virginise and rewrite that ecu for a w210 and also work out the gearbox and can traffic for it.
Thats a headache itself. Easier to just redo this engine