I have seen what i think are civilian models with an XX number before so it may be a civvy version. There are other ways to see if an engine could be a welbike one.
Where the decompresser plug goes (opposite the spark plug) there should be a "Blank" with no decompresser, here is one: http://www.ebay.co.uk/itm/Welbike-Decompressor-Plug-WW2-Para-Folding-Motor-Cycle-Parts-Accessories-/170768360711?pt=UK_Collectables_Militaria_LE&hash=item27c2966507#ht_500wt_1413
I believe the original carbs were brass, the civvy ones were either plated or another metal. On the carb, the srewed on top, if it is a welbike one, should have the grip in a XXX formation, on the civvy ones it was just straight lines lllll. Carps are quite often replaced so it is fairly normal.
Also look for remains of brown or green paint on the engine as the engines were painted in the factories. Try looking with a torch or someting behind the fly wheel, that is the only place where paint remained on mine