I should have said reference to the number ranges in the tables would give you a rough idea of the year. The 1927-29 EW all look virtually similar, and for that matter only differ from the 1925-26 models in small details. The 1929 EW, or A29 as it was cataloged, was in essence a left over 1928 model which used up all the old stock of parts. For example it persisted with the flat tanks, while the more deluxe models started to get the more fashionable saddle tank.
Finding a 350cc EW petrol tank would probably not be difficult, they were a popular machine. However there still is that issue of the frame where it looks to have been modified to accept the saddle tank. The saddle tank might be Douglas, it does seem to have the right shape for the A31 and like Douglas models. Douglas used a common bolt pattern for mounting tanks that would be carried on from model to model, so once the upper frame rail was deal with, mounting the tank would not so difficult. Fixing the frame could be a major project and expense, and that might be the final reason for just leaving it the way it is.