The 635mm size is the old british roadster size, ( 28 X 1 1/2), You can try finding an old raleigh with 28 inch wheels as a parts donor I guess, but that might be difficult. I *think* these size wheels may still be in use in the UK, or at least replacement tires are still offered, I guess you can try finding some online UK bicycle shops and try your luck. They make roadsters in india still that use these tire sizes, I remember coming across a website that sold bicycles and parts- so I wouldnt say impossible to get rims/tires for, just kinda difficult.

Alternatively if you want you can build up some 700c wheels for it, as they are pretty close size wise (the originals would be a tad bigger over 700c though, but not too much).