The build Kurt linked to is an N600 which is a very (VERY) different beast to an S600
If you're genuinely talking about an S600 I'll make a few comments.
- Unless the original motor is completely dead/destroyed, the collectability of the car will be greatly reduced by replacing it
- S600 (and S800) gearboxes are all-indirect so the diff actually turns backwards compared to "normal" cars. If you replace the motor, you need to deal with that somehow, depending on your donor drivetrain (K1200 BMW has been done I believe)
- Replacing the diff in an S600 is a non-trivial task because of the rear suspension design (trailing arm chain drive). It is possible to make a small live-axle fit but you'd need to find something small enough first (possible here in Australia, might be a struggle in the US) and then fabricate trailing arms etc
By way of explanation, I've owned a total of 6 different S600's and S800's over the last 35 years (not all in running condition!) so I've got some idea of what's under them
Good luck with what you're doing but please consider carefully before you cut up what is now an increasingly rare and collectable car.
Dominic