Since Mareks causes tumors on nerves and on organs all over the organs, vets can do a chicken autopsy to look for those. They can tell you what the cause of death was, if it was a disease. They can say if it was possibly Mareks, and there is a test they can later do if you want it to positively...
Here are the links to state vets, how to ship, and some other poultry labs that may do them:
http://www.metzerfarms.com/PoultryLabs.cfm
http://www.usaha.org/Portals/6/StateAnimalHealthOfficials.pdf
Can you place his body in a garbage bag in the refrigerator, not freezer, and contact your state vet for a necropsy? The body can be shipped or delivered. I will give you a link in a few minutes for the state vet. Sorry that you lost him. I didn't mention Mareks since leg injuries are so common...
Roosters can be prone to leg injuries, and high roosts or trying to get away from other roosters could have caused the injury. It can take 3 weeks or longer for a broken bone to heal, and a sprain just as long. Sprains are more common, and the higher up in the leg or hip, the harder to heal, at...
Can you check him for swelling of the leg and toes, and the joints? Jumping from high roosts and horseplay can cause sprains and broken bones. I would place him in a crate or pen with food and water, where he can still be with his flock. Put some poultry vitamins or Nutri-Drench in his water, or...