Actually, the AMC V-8's are about the easiest to swap in a CJ7. The suggestion for a AMC 304 is good. An AMC 360 is even better, and they are fairly plentiful, since they were used in a lot of the AMC cars and trucks. AMC 390 and 401 V-8's are even more power (especially the 401), but they are much rarer. All of those AMC V-8's use the same basic block, with differences in bore and stroke. Accessories, engine mount, and bolt patterns are the same for all of them. With the two larger ones, you'll want a quad carb (or FI), but a MC 2150 2-bbl works fine with the 304 or 360.