There is a documented swap over on Gerry's world for a W201 but he needed to do quite a bit of work to the firewall to get it in IIRC. I would imagine something similar is needed on the W123 due to the size of the engine. The M119 has the LH injection system so you will need to sort all of the computer related issues as well. FWIW I would find a nice Chevy 350 and drop that in if you want a V8 W123. Parts are cheap and plentiful.