Alex,
I do not recall seeing a published weight for the F/G31 models. Douglas catalogs (or period road tests) usually only mention the weights of the lightweight models eligible for a reduced road tax/license. An exception was the 1930s big side valves, the T6 given as 295 and the S5/S6 at 304lbs. Hard to say if the F/G31 would be a little more or a little less. Someone is just going to have to weigh their F/G31 and report.
Same goes for horse power. But since the engine is derived from the Dirt Track engine, you can derive the power from the same source. Not that horsepower ratings for the DT are commonly published. I have a copy of a letter from Eddie Withers were he cites from his notebook on the assembly and testing in June 1928 of DT engine EL1250 in the Douglas Experimental department, where all the OHV engines were assembled before being sent out the the assembly department. He did not mention if it was setup with the in.op.20 or the 10.50.63.20 camshaft, but did mention it was setup at 9:1 compression and presumably running 50-50 petrol-benzol fuel as most DTs were at that time. It recorded 26.65hp at 6200 rpm (it was 500cc.) Since the G31 (600cc) is single carburetor, straight petrol, and I hope you do not spin it quite so fast; you should develop significantly less horsepower. If I had to guess I would say in the range of 20-22hp. Definitely not in the Hayabusa insurance category!
-Doug
