The 2.4 4G64 and the 2.5 4D56T would both bolt up to the same motor mounts and transmission as the 2.6, so eother of those would be a pretty easy swap mechanically. The 4D56 would be easy electronically, but if you enjoy the electrical work then you'd have no problem making a 2.4 work, and parts are easier to come by for it too. The V6 is by far the most powerful engine option, but uses a slightly different transmission than the 4 cylinder engines, so there's that.