I may have some bad news for you. Ask the seller if he has that WD/C engine still. The frame is a post war model C frame which has the same number as a 1943 WD/CO.
The front frame tube from seat to top of engine plates has the tell tale spacer needed for the WD/CO engine. Also the rear frames have no boss for the rear stand as per the WD/C as it had a centre stand. The chain guard is a WD/C item also, as are the forks and I think the front wheel spoke pattern.
so the good news is you can leave as is, restore as a WD/C with a replacement engine, gearbox and rear wheel or restore as a WD/CO by replacing the rear frames (difficult to find).
Please don’t be disheartened as this is how most of us start and we end up with a WD/C, WD/CO, G, J, Jeep, Halftrack, Tank, spitfire and the list goes on.