A 2.3 or 5.0 can be done with *0* body mods. To swap to a 302/5.0 will require V8 motor mounts which are harder than #3!! to find. You may have luck searching Ebay or different classified forums, maybe some scrap yards, but finding them is no easy task. Exhaust can be an issue as well. A 2.3T wouldn't be bad since you already have the 2.3, but be prepared to spend a little bit of $ - power doesn't come cheap!
You
could try for an Ecoboost 2.3, but you'll want all of the wiring and ECM that's associated out of a donor, and that's only guessing that it could be mounted without fabrication of motor mounts or anything else. I don't know if anything exists as far as off the shelf parts that would make it work easily, but it would certainly be interesting! I can only imagine how fun that would be in a II! The Ecoboost sounds pretty respectable in the new Mustang - now install it in II which weighs about 1000 pounds less and let the fun begin! It probably wouldn't be a budget swap though. A Turbo Coupe or SVO 2.3T style Lima 4 would bolt right in....