there is a replacement for displacement, its called boost... if you want instant off idle power use a blower or twin (compound) turbos. If you are not pulling a huge load, drag racing or sled pulling, you will be fine throwing that multi-fuel in the river and installing a stock to mildly modified 5.9 Cummins.. I would not bother with ANY gas motor, the CRD (jeep) VM Motori engine or much of anything else that has been mentioned. There is a HUGE aftermarket for 5.9s, people have put them into everything under the sun, they are cheap and reliable and will last for hundreds of thousands of miles if taken care of...
An HPCR Cummins or Duramax are options as well and there are stand alone engine and transmision management control systems for these and associated parts, but they are not cheap. If you want cheap, get an 89-98 Cummins rebuildable core, find a Cummins/Dodge diesel board that you like and start reading. Talk to any one of dozens of quality shops around the country about the builds they have done for themselves and their customers to get ideas, head off potential headaches, etc.
A 12 valve Cummins with an SAE bolt pattern has almost unlimited possibilites for anything that you could want to do to it or with it. Anything from 160hp with a three speed auto to 1400hp with a 13 speed MD trans and everything in between.
Thats my three cents..
PS: if I HAD to pick something other than a Cummins:
The 93-97 DT466 has a P-pump just like the ISB does. The optional 466E started in 95 and you dont want it. The 84-92 had an A-pump or MW-pump and it can be modded too, just not as common as the P-Pump. The best years for the earlier engine are 89-92, which have aftercooler, roller cam, and big intake opening for lots of power.
However, at 7.6 Liters and over 1500 pounds wet, you can keep it and I will take the 5.9.