Finding a motor that will literally bolt up into your vehicle will probably be close to impossible, but search around. Don't be afraid to use a transmission or transaxle from another vehicle. It is easier to drop the front suspension with engine/transaxle and work at it that way so you can see what bolts into the bottom and make sure to take lots of measurements so when you drop the body back on, it fits. Whatever you do, don't try to pull the motor up and out of that vehicle, it's meant to have the front suspension with motor and transaxle dropped. Shouldn't take but a couple hours do do so and will make this swap nice and easy with lots of room to make adjustments.
A Diesel Mercedes motor from around 1980 would be awesome if you could figure out a transmission combo for it. Don't forget that if you do utilize your current transmission you will need a different torque converter as most diesels have a torque curve far lower than a gas motor. By not swapping converters you will find your motor running excessive RPM's to get moving wasting the fuel you are trying to save money on.
Lastly, realize converting is gonna be costly. Many people (myself included) try to find any way possible to save money during buildups, and often times you will find, a few extra hundred here or there will down the road have saved you thousands if you wouldn't have tried a shortcut that didn't work out on the long haul. Personally, I'd find a good old Mercedes, Toyota, or other model diesel car or small truck and drive it as it is. They are out there, you just have to search harder. I recommend this as sometimes the quirks you get converting will drive you nuts or make it hard for you to ever sell the vehicle if that time ever arrives. Also if you ever break down on the road away from home, it may be difficult to find someone willing to work on if it appears to have been pieced together from different makes and models.
Just my $.02
05 2500HD CC LB LLY, 4x4, 3.73s 235/85R16's, webasto cab heater, to keep it warm.
03 Buick Rendezvous - When you average over 80 miles per day driving around, you need one of these.
85 K-5, 6.2, SM465, Rockwell T221, 1 tons, 36's. More goodies to be installed as time and money allows.
82 K20, 6.2, SM465, NP208, stock except for bed rack, snow plow, and glow plugs are on a toggle switch. It works great for plowing!
72 Postal Jeep - Yet another project