donrae
Rest in Peace -2017

If it's having trouble walking, it's not from living in a cramped cage. Most broilers aren't raised in cages, they're raised in the huge commercial houses. yes, they're cramped for space, but they're able to walk and stretch their legs.
they're what is referred to as a self-terminating cross. Meaning, they don't really live long enough to reproduce themselves. If it's already having leg issues, I'm thinking it's not long for this world. Personally, I'd butcher it and enjoy the free dinner

Some folks have kept meaties as pets, and nursed them along for several months or a year. But, those are usually folks who had the chicks from hatch and limited feed while free ranging and encouraging exercise. Getting a bird that's been raised expressly for slaughter at this age, I'm thinking it's not going to live very long, sorry.
I don't think there's much risk to your current flock. The commercial broiler industry is heavily invested in keeping their birds healthy. I do think the broiler is probably at risk of cocci, etc one it's on your ground for a week or so unless you feed medicated feed or dose with Corid.