Hello Ruurd
I too am busy restoring an EW and, because mine was left out in the rain for forty years, I have had to find lots of parts.
Firstly, you should join the London Douglas Motorcycle Club (LDMCC). The pre war spares scheme is very good, and can supply you with most of the items which you would expect to replace through the life of the machine; springs, valve guides, pistons, even valves. But the list is not comprehensive, and the bigger items you will have to find yourself.
There are plenty suppliers for standard parts like chains, handlebar levers, mudguards, carburettors, magnetos etc, and at least two manufacturers listing EW exhaust systems. The area you will find the most difficulty will be the bits that are unique to the EW like petrol tank and footboards. Used engines and gearboxes are fairly easy to find.
I have found that wanted adverts in the LDMCC magazine usually work and the parts section of this forum sometimes has EW parts for sale. The EW was quite a popular model in it's day, so the parts are out there, but you sometimes have to be patient.
I have a few bits and pieces for sale, so if there is anything you specifically need, let me know.
Good luck with your project,
Stuart.