My friend has a 1998 Montero Sport, the engine went out and he had it replaced with a 2000 model engine, will this work right

It's true it can work as long as the mounts are the same, harness I believe work and computer is the same

Ok, I have another question too, you said the computer needs to be the same, the same as the new engine , or refurbished engine, if getting the computer for that particular engine is not possible, what would be the next logical step

Well normally the computer you have should work. But if it different you can tell by the harness or part # on computer. 60% of the time computer fits/matches

Is the engine model the same? But just different year?

Yes, the engine is the same, just the 2 year model difference, now the engine will turn over, it just doesn't start, but it does turn over, and the battery has a full charge.

Ok, I just found out that it is the same engine type, but there is one difference, the air intake is different on the 2000 engine than the 1998 engine,

Ok that's the least. Here's what you need to check. Is fuel coming up? Are the injectors pulsing and timing ok?

I just realized that the engine has already been installed.

Either way one of those things are hindering the engine from starting. Find the problem and I can give some solutions

Ok, so according to the mechanic that installed the engine that the place he got it from had checked the timing before it was purchased, would that still be an issue before or after it was installed by the mechanic?

Yes the timing is very important before and after. Normally when a mechanic install an engine we routinely check sensors,timing, etc to make sure nothing is wrong prior to installation. These things are easier outside of car rather than repair done inside.

Now knowing that..how do you engine is good? Did they bench start it?? (Start and listen to engine without installing engine in car)

Just like when your buying eggs from a market.. you check to make sure none are cracked or missing