Whatever engine you manage to fit, make sure you use an offroad carburetor. An ordinary carburetor may well work fine on the road. On steep inclines, however, it can do all sorts of funny things such as stalling or flooding the engine as well as less funny hings such as setting the vehicle on fire. An offroad carburetor is designed to operate at extreme inclines, an ordinary carburetor will however leak fuel to the outside when inclined more than about thirty degrees. Since the exhaust manifold is right below the carburetor this will very likely lead to a serious fire.
The standard Unimog engines (180 & 130) have two struts attached between the sump and the bellhousing. These are extremely important. If you fit a non-unimog engine (be it Mercedes or otherwise) you will most likely not have an attachement point on the sump for these struts. Such an attachement point MUST be made and it must be very strong. The two struts help to keep the engine and gearbox aligned in a vertical plane, without them, the engine and bellhousing would only be held together by the 6mm (quarter-inch) adapter plate which flexes much more than you may expect. If you are using a Mercedes engine then you can probably re-use the original Unimog sump (Christian Falzon did for the 123 engine). Luke Miller reports:
The best swaps are those using the Mercedes-Benz motors. It's Mercedes truck - keep a Mercedes motor in it. (credit to Jim Ince there). I'm putting the M108 motor in my truck. I pulled it out of a '67 4-door 250S sedan. The other good motor for the mog is the M130 ('68-'72) (not sure exactly which cars this can be found in - 280 sedans I think). The M108 has 140hp and 220 ft lbs of torque. Throw a supercharger on it if you want more oompf! A supercharger can raise hp and torque40-50%. Keep the oil coolers on these motors as they run a little hotter than the M180. Most of these motors have dual carbs and require a little modification to the passenger side engine cover and foot-well. Also, the hole on the flywheel are slightly smaller and must be bored out. (more on that later as I get into that part of the conversion)
M 180: 2.2 liter gasoline engine (2196 cc displacement) 6 cyls inline, 4 stroke with standard 7.0:1 compression: 82hp @ 4850rpm, 105 ft-lb @ 3200rpm With high compression heads: 8.7:1 single carb 105hp 8.7:1 dual carb 124hp Used in the following MB cars: (info, please) OK to use from car models up to 1975. Engine numbers and Unimog models: 180.928 404.1 180.953 404.0 180.958 404.0 M 130: 2.8 liter gasoline engine 6 cyls inline, 4 stroke with standard 7.8:1 compression: 110hp @ 4800rpm, 137 lb-ft @ 3200 rpm with high compression heads 8.7:1 dual carb 146hp 9.0:1 dual carb 160hp 9.5:1 dual carb 180hp 9.5:1 fuel inj. 185hp (non-standard conversion) 7 main bearings single overhead cam, inline 6 2 two barrel (Zenith?) carbs or Bosch fuel injection found in the Mercedes-Benz 250C, OK to use from car models up to 1972. Engine numbers and Unimog models: 130.925 404.0