You will have to replace the tranny as well. The AX-5/AX-4 verity transmissions wont live long behind a 6 or 8 cyl. engine. Besides I am not sure that there is an adapter for that application(check Advance Adapters
to make sure).
To do the I6 swap I would start looking for a Cherokee for swap material. B/c you will have to replace the tranny as well get the whole shooting match from the same vehicle, so you will know for sure that it will all match up. I personally would look for a wrecked Cherokee, they are plentiful. Get everything you need and remember to get one from a 4wd(that way you won't have to purchace the x-fer case adaptor that is already on the 4wd models). You should be able to find the complete engine, computer, and tranny for just a hair over $2,000.
If you have any inclinations to a V8 swap later on down the line, then this is your time. A wrecked TBI 350 chevy 4wd pickup is a great swap, and plentiful too(driving the price down somewhat). Get the engine w/ all assories, computer, tranny(the 700R4 is a good tranny) and you should be set. Should be able to find it all for less than $2,500 if you look enough. Those engines will run forever and there is so much aftermarket for them.
Also, if you dedide to do the swap(either to a 6 or 8 cyl. then you will need a new input gear for your NP231 x-fer case(yours, behind the 4 cyl. is a 21 spline, most 6 and 8 cyl. use a 23 spline input gear.
If you do a swap, either way, you will have engine mounts, large mechanics bill, and misc. other charges that you will incur. Other possibilities include: shifter for auto trans, new radiator, new hoses, new belts, etc.
Good luck, and get that thing back on/off the road.
llll '90 YJ, NV4500, XD9000i, 35" MT/R, Snorkle, 4.5" RE lift, Tera Revolvers, etc. etc.