Very few Villiers powered beasties command any sort of serious money which is a pity as I have a couple of them.
Your bike is easy to date from the frame and engine number and the VMCC Library can provide copies of the spare parts list, drivers handbook and whatever other info is available.
There is no specific source of CE spares and you will just have to haunt swap meets to find the missing tank, gearbox, primary chain case etc. Coventry Eagle were a made in goodly numbers and the survival rate is pretty good so you have a fair chance of finding everything over time.
The 250 Villiers engine will get you around on a vintage rally at a respectable rate and can be made surprisingly reliable. Villiers engine spares are much easier to find. All told a good quality working mans bike that will give you good service.
Cheers.