i'm pretty sure thats a 250. Oil filler necks were longer, as far as i know, on the 250s & the model G de-luxes but not on standard model Gs. The longer filler neck &/or extension was necessary when the engine was mounted in a sprung frame(250 Clipper & Model G de-luxe).
Cylinder head on the 250s looked very similar to that of the Gs but was different. The one in your photo is certainly different to the 350 ones i have. Hope that helps
Your engine is an interesting collection of parts. Basically a 350 Clipper motor motor as it has the mounting for the alternator alloy primary cases and longer mainshaft for the rotor but it has a prewar cylinder head. Enfield didn't use screw in intake tubes and clip mount carbs post WW2, unless they were building Parts Bin Specials for particular orders. Interestly it has the timing chest machined that would allow a magneto or even a magdyno in place of just points ignition. It would be interesting to know what the engine number is. Cheers.