Sure. Why wouldn't you be able to?
The older the bird, the tougher the meat though. You could always brine the meat for a few days first to tenderize it if it's more than a few months old.
For the Chicken (cockerel) Fajita's I think that I will try this recipe for them:
https://www.homesicktexan.com/2011/09/chicken-fajitas-recipe.html