G Wagon

Why?

Or do you really mean 100000k (which in real money is 100 MILLION and even I have to admit that is a lot of miles)?

If you actually mean 100,000 then that's nothing to worry about.

I'd second that.

Unless you have a specific problem, which in many cases is simple to fix (valve-stem seals, head gasket, timing-chain cover leak), I wouldn't dream of changing the engine. Many 'replacement engines' are just taken as-is from scrap yards and tied to warranties, which require you to return to the same (cowboy) garage if anything goes wrong - which it most certainly will. If you are referring to 100,000 miles, that's barely run-in for one of these.

If you have some money to spend, check the cooling system and budget for a new OEM Behr radiator (from Northern Radiators), water pump,  thermostat and hoses, if still on the original items. Then, replace the ignition leads, spark-plugs, distributor cap and rotor with Bosch or Beru items.... and a new Bosch ignition-coil, as if this fails due to age, it will take-out the Siemens EZL ignition module, which is £££££ to replace new.

When you've done all that, check, lubricate and adjust throttle linkages and it should run like new.

Of course, you should change all oils and fluids and filters (engine, gearbox, front and rear diffs, transfer case, brakes, clutch, diff-locks etc)

If you still have the original exhaust system, note that the catalytic converter is eventually prone to blocking-up and will reduce engine performance. A new complete exhaust system from MB is around £3,000, but you can have one fabricated for much, much, less. A metal-matrix catalyst would be better than the standard ceramic block.