mid-later 60's Is what I have seen.
I found this a few years ago for them being sold out of a 1965 Bennet Bros. catalog:
I don't know if there are ways to decipher the numbers on the Clinton engines to verify manufacturing dates. That minibike/company seemed to morph into a company called Manco closer to 1970. You can still see a photo of that bike (with sidecar) in a brochure photo for the Manco line but only offering the sidecar.
There are usually a fair amount of Clinton parts on ebay, I dont follow them too closely to know of any good sources for parts though, sorry.
Its extremely hard to put values on mini bikes especially lesser seen versions such as yours.
Good luck with it, Looks to be overall not too bad of shape and still pretty original. I think the Manco type chain guards they had are available as reproduction now if I am not mistaken