I think you will find that the screw threads on those SMLE (No.1) are really funky - not the same as even the No. 4 Lee Enfield - might have been British Enfield series profile or similar. Likely you will want to find that "part" versus find a screw that would fit to that thread.
Might even be an "iffy" thing to have the barrel clamped down to the forearm - for sure there is a made-in-1955 Winchester Model 70 here that was made with similar - a screw up through the forearm into the barrel - goes about totally against the idea of a "free floated" barrel, I think. But, perhaps that missing screw went along with a plan for the original long stock, which is also gone. I believe the original stocking criteria had the rear end anchored at the receiver, then some "up" pressure near the muzzle, and then that barrel band to pull down on the barrel between. But I do not know what was the plan when that long stock is cut off? Likely a thing to actually go shoot it at targets and see if it works as is?