I would take a look at this feature request, as there are also some other users who share the same desire as you for the Decathlon. Keep this in mind though…
As Infinite Flight transitions into an era where aircraft are extremely detailed (for example, the moving cockpit displays), they likely aren’t aiming for a huge difference in aircraft quality across the board; like Jason said, the Decathlon was not very detailed at all. Rather than divert additional resources to bring that aircraft up to current standards, it was simply replaced with the XCub.
For additional information on the XCub replacing the Decathlon, you can also see this post:
