Getting back to basics for a minute, it's bigger, has 2 more HD, 3 better AC, is faster on land, and slower in air.
A standard wyvern shrunk from G to L in 3e. A wyvern advanced to 9 HD would be Huge, which I think is appropriate here.
So, take a standard wyvern, advanced to 9 HD and Huge, keep Dex unchanged, and modify speed and reduce maneuverability to clumsy?
Thus...
Huge Dragon
Hit Dice: 9d12+36 (94 hp)
Initiative: +1
Speed: 30 ft. (6 squares), fly 50 ft. (clumsy)
Armor Class: 20 (–2 size, +1 Dex, +11 natural), touch 9, flat-footed 19
Base Attack/Grapple: +9/+25
Attack: Sting +15 melee (1d8+8 plus poison) or talon +15 melee (3d6+8) or bite +15 melee (3d8+8)
Full Attack: Sting +15 melee (1d8+8 plus poison) and bite +13 melee (3d8+8) and 2 wings +13 melee (2d8+4) and 2 talons +13 melee (3d6+8)
Space/Reach: 15 ft./10 ft.
Special Attacks: Poison, improved grab
Special Qualities: Darkvision 60 ft., immunity to sleep and paralysis, low-light vision, scent
Saves: Fort +10, Ref +7, Will +7
Abilities: Str 27, Dex 12, Con 19, Int 6, Wis 12, Cha 9
Skills: Hide +7, Listen +13, Move Silently +11, Spot +16, 8 more
Feats: Ability Focus (poison), Alertness, Flyby Attack, Multiattack (B), 1 more
Environment: Warm hills
Organization: Solitary
Challenge Rating: x
Treasure: Standard
Alignment: Always neutral evil?
Advancement: 10 HD (Huge); 11–21 HD (Gargantuan)?
Level Adjustment: —