missing PCV Valve?

[cowking7]

hey everyone, I need some help with my pcv valve. the port on my right manifold was plugged by the previouse owner. is that ok? I think the one on my left manifold is hooked up. is only one necisary? i dont know a whole lot about them and just thought someone could explain to me how it all worked, also the plug on the right fell out and melted on my exhaust, do I need to get a replacement plug? im guessing its a good idea since thats a vacuume leak but how urgent is it?

thanks