Welcome to the asylum, here we all speak your addiction

For some basics; pretty much all parts are interchangeable between Scout II's from the first one like you have (does the sticker in the glovebox say 'Scout 810'? Those are the very first Scout II's) till the last one in 1980. Obviously some things are different- the Scout II's that had AMC I-6's in them used different motor mounts on the frame, the wiring was changed somewhere around 1978 to a better system than the older Scout II's used, axles and transfer cases were changed during the model run- but most parts will be the same. A hood from a 1980 will slip right on your 71, as would a top, quarter panels, front fenders, all of it. A 345 from a 1980 is a drop in swap, and you can get a T-19 with bellhousing from a later model and slam it right in with minimal changing. There are guys on here who have gone the Chevy route, and they're happy with it, but most of us stick to IH power. The IH engine is a medium truck engine in a light vehicle- as such, with proper maintenance it's unlikely you'll ever wear one out. Swapping to a Chevy will take adapters for the bellhousing to engine ($$$$$), and I think the consensus is if you want to go that route go GM all the way- engine, trans and transfer case to keep it simple. You'll still have to redo the frame engine mounts, crossmembers, etc., but it will be cheaper than the adapter. What makes you think your IH engine is in need of a rebuild? Lots of times, issues with these engines are caused by people not knowing the little 'quirks', such as being timed off of #8 cylinder instead of the usual #1. Post on the Tech Talk forum any problems you have, and people will jump on and help you out- searching the forum is also very productive, and just browsing for things that 'sound' in the title like your problem can keep you reading for hours and give you a pretty good idea of how the site works and a basic understanding of these engines. Again, welcome!