My recommendations will be as follows for your Jeep.
Toyota 2L-T For this Jeep I can easily recommend Toyota 2L-T This turbocharged engine is available in many variations. Although some are EFI and others have ceramic valve seats, power output is roughly the same in all models. this is the best engine for flat fender Jeeps as far as performance is concerned. It can be easily squeezed into the engine bay on these Jeeps. Being turbocharged, they have potential for even more power in the hands of those people who know what they’re doing.
3C-TE (2.2 Turbo Diesel):
4 cylinder turbocharged EFI
Displacement= 2184 cc (Engine configuration: horizontal rear wheel drive )
The very fact that the engine lies flat in the engine bay instead of sitting vertically like a normal engine as we're all used to raises some doubts as to whether it will even fit between the fenders of the average jeep.
Power output for the non-EFI version is actually slightly higher than the 2LT but it will require slightly lower gearing because it needs to rev higher. The EFI version is even more powerful.
Overall, my opinion is that IF the engine can be mounted safely and out of harms way in a jeep, then it should make a good powerplant for a light vehicle like an M38 or M38-A1. 3C-TE can be used an alternate, no doubt about it’s performance but for UR Jeep I will recommend 2L-T.
Nissan RD28: (Not Recommended) This peppy, high revving, 2.8 liter 6-cylinder engine has found its way into quite a few CJ-7s. The 6-cylinder configuration makes it exceptionally smooth for a diesel. Its high revving nature means plenty of horsepower at the top end of the RPM range, but small displacement takes away low end torque. This is a good engine for daily driving on the street, but not ideally suited to off road situations where low end torque really counts. This is the main reason why the technically inferior but torqueier B series engines have always been preferred over the RD28.