New to the forum so please be kind

I've just bought a Jimny '02 Manual on a private sale, no come backs.

I've notice a loud "clunk" on take up and over-run and when rocking the car in gear. I've traced it to the transfer box.
Is this common and is it likely to give up the ghost rapidly.
A bit of research leads me to think it's the chain. Is it possible to replace just the chain or is it cogs as well?
So many questions, sorry but one more, where's a good source for the bits I'll need if I go for a rebuild rather than second hand.
Thanks
Peter