As Bill noted, if you plan on racing, check with your local organization or club for their engine rules.

If this is just for sport, no problem - pick an engine you are comfortable with.

Any of the engines you noted will work well. The Rossi .40 is a good value these days.

The only concern with the Rossi is that it is heavy, and you will end up with a nose-heavy aircraft - its hard to balance a Q-500 with a great deal of mass no the nose. Also make sure to support the tail end of the muffler.

The best 'econo-speed' setup you can put on the nose, is a Thunder Tiger .46, with a Jett-Stream muffler added. For about $140 you will have one heck of an engine. Use a 8.75x8W prop and you will have all the performance you might want, with limited expendature.

For a Jett product, the standard SJ-46 or SJ-50 is the way to go if you are just flying for sport. .... and if you do want more, the Jett SJ-50 with an SS muffler is the way to go. Similar performance to the Q-500 .40 racing version, but throttles and idles well - and is very user friendly.

You will want a backplate mount for any engine you chose. Jett offers replacement backplate mounts for TT, Jett, OS, and a few others.

LCSPERFORMANCE.COM offers backplate add-on plates that are sometimes more conveninent.

Take the time to search the Pylon Q-500 and Extream Speed forums for modification/reinforcement information on the Viper.

I hope this is helpful.

Bob