This bike rather makes me think of Ogri's mate Malcolm's 'Norumph' built out of the bits left over from everybody else's Tritons. Didn't Ogri say "God's teeth, Malcolm - What's that ?" and the small bird on the fence said "Don't forget the dexion bits".
It is possible to build a reasonable WD bike around an engine or indeed a rolling chassis but I'd hesitate to try it without access to UK jumbles and a sufficiently large network of friends and acquaintances who merely have to be convinced that they're sitting on parts that they don't really need.
Are you in the US, Scott ? I can imagine that rigid, girder forked British bikes are fairly unusual there and the lack of alternative sources might make a project more tempting but against that, you'll find local parts supply almost non-existent and you'll need to be very wary of internet purchases.
It's a matter of historical fact that a good number of WD bikes ended up on the Indian sub-continent and there is therefore a small industry devoted to keeping those bikes running. Unfortunately, the quality of those parts is often abysmal and they are now expecting western prices for them.
I have to say that I really don't see this one as a first project unless you have access to a machine shop and intend to make the missing parts yourself.
If you didn't pay too much and the import costs weren't too high then I too would be tempted to recommend identifying the parts properly and putting them on eBay. You should find that the profit goes a good way towards finding a more feasible first-time project.
There are quite a few WDM20s in the US. It seems that several containers full of ex-BAOR machines were imorted by a dealer in the 1970s and many of them have seen little use. If you really want an M20 then keeping your ear to the ground when one of those comes up is probably a better option.