I’m slowly in the process of converting the old creature codex from the MB to BWGR’s updated system, but I’m also of the opinion that using traits to represent everything different / interesting about a mob is a bad way of handling it because it means a lot of looking stuff up in the middle of trying to run an encounter.
My plan with my list to to have the stat block as normal but then also include a bullet point list summary and simplification of their most useful powers / traits, that aren’t fully explained by the statblock, and what direct mechanical effects they have.
So in the case of the Wyvern, I’d write something like this:
- Dragon’s Breath Tests Forte to use: margin either causes Tax or reduces the IMS on the weapon’s next use for 10 hours, dragon’s choice; while the Mark damage is at zero the weapon becomes unusable. Range as Pistol. IMS = DoF (1-2 / 3-4 / 5-6).
- Tunnel Vision Reduce vision penalties for darkness by one step.
FYI, don’t throw a Wyvern at the party in the first few sessions. They’ll die 
To answer your other question, some of the traits from the MB made it into the Codex, but not all of them.
[Also, worth noting that what I’ve written there isn’t exactly as the Dragon’s Breath trait is written in the MB. I think it’s good to make certain simplifications; partly because certain things have clearly changed—like the fact that this version of dragon’s breath clearly has a VA, whereas the original ignored armour—and partly because the original version took up almost half a page to fully explain itself, and nobody’s got time for that. GMs have got enough on their on their plates already].