If you put in a TD42 it will loose the acceleration edge and will loose a little bit of torque as well. The TD42 is a simple reliable yet inefficient engine. You do things to it that one can only imagine and yet it will run like a dream. Lasts quite long as well. A properly maintained engine can go upto 300,000 kms.
I think your best option is to get that RD28ETi fixed and go with it. Swapping a engine is one pain you don't want to take, there are multiple things that can go wrong at any point. Initially it seems easy but when you get into it you realize that you were better off with the original one. If you swap the engine you'll need to change the transmission as well because its no good with high power engines.
The best option in terms of a swap is a Nissan ZD30DDTi with a S50B transmission. The transmission is bolt on and you will just need to replace the engine mounts as this setup comes standard in Y61 Patrols/Safaris. I have rarely come across ZD30 is Pakistan though.