I found 13.5" to be just about max length for a CFX. I had the Leapers 3-12x44 full size on mine, big and heavy, and I had issues with Leapers scopes in general, but a good match to the CFX. I tried a Simmons 44 Mag, love the scope, it's currently on my Talon, but it was too long for the CFX. I just picked up one of the Centerpoint 4-16x40's at Walmart for my B50, seems like an okay scope for $70 but I think it would also be too long for a CFX. Leapers and Centerpoint are EXACTLY the same scopes, BTW, just a different name. The Leapers compact 3-12x44 would certainly fit with no problem, but I've had problems with it. I think the optics on the full size are far superior. You might also consider the Nikko Stirling line of scopes. The Platinum Nighteater 3-10x42 (?) is actually longer than 13.5", but because the front objective is slash cut, it fits fine. Not a bad scope for the money, but not great. Plus it's not available with a mil dot reticle. The Air Kings do have a mil dot, but I don't really know anything else about them.
For a 1 pc. mount, you can't go wrong with the Accushot mounts. Another real good choice, but more expensive, would be the BKL mounts. I feel that the CFX benefits from a drooper mount, I'd use a BKL 260 D7. Hope that helps, and good luck.
Dave