Here's a thought..... Buy the best sticky tyres you can get, drive your cobra often and hard and wear them out while having fun. The tyres will only last a year or two. Problem solved!
In all seriousness, old tyres mean less grip, whatever the condition. Not worth it (especially with a cobra). Anything over 10 years old, definitely not worth it - see article on Inquest re M5 accident where 5 people lost their lives, 18 year old tyres were the cause....... I seem to remember reading a post on here saying one of our members was involved, making it especially sad. Nothing more to say..