They don't need heat as long as they have adequate ventilation and reasonable draft protection, but maybe you need to provide it for you. A lot of what people do for their pet chickens are for the person's benefit. The chickens don't care and don't need a lot of it.
No matter what you do, they do need ventilation over their heads or you need to clean the poop out real regular. Their poop creates ammonia. Their respiratory system is fairly delicate and can be damaged by breathing ammonia. Ammonia is lighter than air so it will rise. You need ventilation for the ammonia to escape that is higher than their heads or you need ot be sure that the poop does not build up enough to allow ammonia.
I don't heat my coop. I prefer to not put my chickens in danger. If the power goes out and they are not acclimated to the cold, they could be in trouble. But that's your choice.
Fire is also a recognizable danger. You'll read posts on here where people have burned the coop down heating it. That's a shame since the chickens probably didn't need the heat to start with.
I suggest you look for some type of heating device other than a heat lamp. Any type of electric device creates some risk, but some are a whole lot safer than others. Try to find a radiator-type heating device or something that does not get all that hot. If you install it properly the chickens are not going to get burned if they bump up against it and it should never get hot enough to start a fire. You probably have a lot of easily-combustable material in that coop. Get something that does not get hot enough to catch that bedding on fire if they scratch some of it on the heating device. It might be beneficial to box or cage it in so they cannot roost or poop on it.
I don't have any specdific recommendations for make and model but if I were going to put a heating device in there, that's what I'd be looking for.
Good luck!
