A Cummins 5.9 "B" series will fit right in. The early Cummins 5.9 with the 12 valve heads is about 250 pounds lighter than the later 24 valve. Even at the lower weight the early Cummins is about 850 pounds.
If you make the decision to install a 6.9 or later 7.3 V8 Diesel, you will have to rebuild the cab around the engine. I built a "doghouse" set back of 6 inches to allow for the width of the engine. The 6.9 and 7.3 are 37 inches wide. The left inner fender apron must be modified to accommodate the V8 Diesel. I made this modification to my R120 in 1991. The performance was a little disappointing.
The 6.9 and 7.3 engines are very heavy. At 1150 pounds, I had to build a subframe and add air springs to carry the weight of the engine.
Cooling was a situation. I had a custom radiator built for my R120. The radiator was set forward as far as was possible. The new radiator was 3 inches wider than the OEM. The new radiator cost $1000 before tax.
These older IDI engines are close to obsolete now and some parts are becoming difficult to buy and the prices of parts are going up fast.
If you can find a Cummins 4bta it will be a better fit for your situation. The 4bta weighs the same as the OEM engine, has double the power and triple the economy.
I have a Cummins 4bta with a TH475 and an NVG 241c transfercase. They are attached and work together. This package was destined for an L110 4X4. I am getting older and I have to consider my options. This engine package may be for sale. I do not know when this might happen.
https://youtu.be/_U5M6eIQpZk