A DLE 55 RA can be put in your cowl very cleanly, other rear carb'ed engines will fit also but know that a side muffler will fit also. Tower Hobby has a side muffler that fits a DLE 55 side exhaust very nice, made by J-tech and I have one in a 1/5 scale TF Mustang that's smaller at the cowl than that of the 1/3 scale J3.
Leroy