I. Dont know anything about isuzu's am more of a Honda guy. But i have a 98 rodeo and my head gaskets broke due to overheating. Idk to either replacing. Head gaskets or swapping motor.
Can a 02 rodeo motor fit in a 98 rodeo..

 

If the rings are still sealing well, and theres not abnormal noise coming from the block, id just do the headgaskets...But be prepared! It is a pretty decent amount of work.
Im a honda guy as well, but if you work on hondas and are semi-mechanically inclined, you should be able to tackle some v6 dohc heagaskets.

Im replacing he HGs on my dads 98 rodeo. Just pulled the intake manifold and radiator assembly last night. I can walk you through it as I do mine over the next several weeks. Took a few pics for reference...

If you repair/not replace , just remember to stay organized. Take some pix to help you remember if you have to.
Good luck!