Best bet: Find a 2.5 in a junk yard, do a 4.0L swap, or figure out how to swap that 4.2L in. For the 4.2L, I'd invest in a Weber carb, all new gaskets, and redo much of the wiring. Most of the time, Comanche's were neglected and the bodies were rotted out, and deposited at the local bone yard. Finding a clean one is tough. However, they still might have viable mechanical parts.
If yours is clean enough, I'd consider either of the three options really. If you can fit the 4.2L in with minimal work and get the motor for free or cheap, you'll have less electronics to deal with, which also means less wiring to deal with when it comes to the swap.
From my research, the 2.5L is a good motor. It's slow, not powerful, but its great for someone who doesn't need speed or power. If you want a Point A to Point B method, perhaps finding a running 2.5L is a good option for you.