No this is not a "big single" that refers to the BSA Gold Star models ZB, BB, DBD etc but I would say that it is a good bike to restore. There does not seem to be many parts missing and a single is a fairly easy restoration. If it is a B40 the frame number and engine number will begin B40 ...... There are plenty of spare parts available, many are being re made, but get them from reputable source. The colour is up to you, but this depends if you want it to be completely original or not. If so I think that year B40 was black or red only. All the best.