The SR-71 is a retired plane. They don't even use it anymore. It doesn't even have a weapons system. If they're going to go with a mothballed airplane, they should use the Grumman F-14 Tomcat. It's the same plane from Top Gun, and it is the airplane the original G1 toy was based off of.
If they want something more modern, they could use the Navy's F/A 18 Super Hornet, which replaced the Tomcat. The point of transforming is so they can blend in. You can't blend in very well with a plane that isn't in use can you?
Check out the Super Hornet!