You would need the transmission (possibly x-fer case(?)), drive shafts comes to mind as possibly needed tweaked, engine mounts, spice in the 2.0L engine mangement, exhaust, etc... Would be a fairly big job. I would rather go for w/ an EFI 2.3L if you're set on an I4, but that's just an opinion.
Also had a carbed 2.8L that would work with your 2.9L transmission. Engine management would be fairly simple to splice in. 2.8L had siamesed exhaust ports, so the 2.9L exhaust manifolds would not work. If you have an A4LD automatic behind your 2.9L, this has an additional lock up circuit from the computer that runs to the NSS/back up light switch. You would need to figure something out for that.
If you've just noticed coolant in your oil, could be a cracked head, which is much more common that blowing head gaskets with this engine. Unfortunately there's no cheap fix either way. I would hate to recommend another 2.9L because most of them are problematic, but on the other hand, others have gone 300K+. Maintenance is a big key.