If you have enough money, anything can "fit". That said, how much money do you have?
Seriously, the W124 engine bay is pretty versatile, just look at the 500E for instance. The serious issues arise with engine controls, drivetrain, a/c, and suspension weight, and other niggly little details. On the non-MB route, how about a twin turbo Toyota Supra engine and trans? Supercharged Ford? Or consider the sound of a Dodge 340 with a "six pack" or, dare I say it . . . a Hemi!