If you want to try to repair it, you'll need to try to restore the spring action that holds it closed, and you'll want to lap it with valve lapping compound. Lots of valve lapping videos online, not for this woped specifically but enough to give you an idea of what you need. pull it out with I think it's just a 15mm wrench and you can look at it and work on it.
Plugging it doesn't really remove the originality, you could do it just to test if it makes an improvement and keep the original valve around to swap back on whenever. I think you can buy replacement valves if you email doscycles.com they might have some (probably not listed online but they have lots of back stock stuff that's not listed on their site) and there's also mopedjunkyard. Usually treatland.tv and doscycles are best sources for parts, but treats I don't think has much stock replacements. 1977mopeds is best to avoid anymore, their service has failed. mopedjunkyard is typically pricier but often has more original parts.