Well, you could always Scale the Cockatrice. As a Magical Beast, +6 hit dice is +2 CR; +9 hit dice is +3 CR, and the size increase at Hit die 9 is +1 CR as well. Oh yes, and do note that the save DC is 10+1/2 hit dice+ability modifier (Con) - so if you add 9 hit dice to a Cockatrice, the save DC goes from 12 (10+1/2*5+0) to 17 (10+1/2 * 14 + 0).I'm trying to scale up an adventure that uses Cockatrices. I'm not familiar with advancing monsters, so I'm wondering if there is an alternative creature that is slightly more powerful (maybe 2-4 CRs higher) than a Cockatrice that petrifies victims. Any suggestions?

Nice....bullsh*t...Gygax...bastardising legends...lack of imagination...Gygax was to mythology what Hollywood is to World War II...

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.