I thought something like that might do Mick.
Standing back and looking at this like the sort of projects I seem to end up with, Id be trying to find out a few things. I assume Vic that you have no idea whether the gun was working or not before the end cap went AWOL?
If that is the case, Id assume you want to roughly put the gun together to see if there any other problems?
A couple of things Id try/ or appeal for are (1) if somebody actually knows the size and thread of the cap---which would help for looking for something like Mick has suggested (2) if anyone who has a gun lives close enough to you for you to visit or them to visit you, to try if their cap would fit your gun Ie, your threads are ok. (3) failing the above, if someone could measure both the OD and the Depth of a cap to give someone a clue , for if they have an unknown one in a spares drawer.
I wouldnt be worried at all about a guide fitted to a cap as you could always sort something like that later.
Im surprised someone hasnt got a dead gun knocking about which would give you a chance of putting something together out of the 2.
Often with these things you do not want to spend much at all until you know you are in with a fighting chance of getting a working gun together-------then there is the problem of would getting the bits be dearer than getting a working gun.
Good luck with it