It's an interesting project Paul, but as Len points out something weird has happened to the frame.
It's worth having a look at the frames from the 1910-1911 models - luckily Michiel posted some very clear illustrations here:
https://www.douglasmotorcycles.net/index.php?board=51.0In 1911 there were a mix of the 1910-style frames that Len refers to, and the Model E with the frame rail under the tank and lower engine position. I agree with Len that the frame looks like one of the 1910-11 frames forced into submission to get the engine lower. Of course this could have been done in 1914, 1994, or 2014!
The date of conversion is important, I think. If the bike was originally, say, a 1911 Model D, modified a couple of years later to lower the engine, I'd be inclined to leave it as it is, bent saddle tube and all. But if the frame was cobbled up in recent years, I'd be inclined to modify it back to one of the original frame designs, either Model C or Model D.
The "pedal lug" doesn't look much like the ones used on the early model Douglases, but this needs to be investigated further.
I don't suppose there's a frame number anywhere?
Cheers
Leon