Well, from what I have read, it is possible in an old(er) bird if the virus was latent, it's just rare. My vet who's and avian vet is not hopeful, but I need to talk with him. My roo did have the kind of lymphoma you find with Marek's. What I don't understand is that if my birds all have Marek's, would there be a steady decline? I've read that some birds die with no symptoms. But mine do look pretty darn good. They are eating now, but not their feed. The one who got floppy is a bit slower and doesn't eat as much. But - I got an egg today!! An egg. Haven't seen one of those for at least 5-6 weeks. I guess it's a waiting game, maybe some blood work on the slow one.
Anyway, I do wish you the very best. I hope you get a great bunch of new birds and they stay healthy and happy for a very long time!