RE: Which engine for GP Viper?
The .50 will be fine. Its the same case as the .40 and .46 so size is not an issue.
I don't believe the sport .50 will be as fast as the Jett Quickee or Nelson .40 engines. However, it will allow you to fly at speeds other than full throttle.
I have a Jett .50 on a .40 sized kyosho Spit, and its a ball to fly.