The easiest way would be to provide a few good photos here and someone could ID it. Lacking that, the 61 has DLE61 cast in the housing just above the carb. I'm not sure about the 55 but there are probably some casting marks on that engine for ID. The 61 also has curved surfaces on the cylinder to lower housing mating area. The side exhaust 55 has a square mating flange area. Not sure about the 55 rear exhaust but its pretty easy to ID that engine because they don't make a 61 RE. You can also go to Tower Hobbies or Valley View RC websites and look at the 3 views and photos of the engines. You can quickly ID the engines there. There ya go .... several ways to ID the engine.