Q: will crank but wont start

the 96 engine was put in a 99 key was misplaced mechanic who put it in has not come back went to dealer to get a key using the vin on the truck got vehicle to crank but wont start. plugged in a coder and came back A Bus that the computer is fried. Got a computer for 96 dodge ram 2500 v10 put in same thing came up. The new person that I had put in the computer from the salvage yard is at a loss. I have researched a little and think that we didnt match numbers of the truck vin that the 96 came out of to the computer we got and maybe something with the starter and key I got from the dealer with the vin on the 99 truck. Dont know where the mechanic that put the 96 engine in the 99 left off got the same A Bus with the replacement computer What needs to be done. The second mechanic says he checked fuses crank sensor and also battery sounds like its going dead. Dont think this mechanic wants to figure it out. What needs to be done.

My car has 88000 miles.
My car has an automatic transmission.