Re: Pre-Unit Case "use or not"
that piece that is missing really has very little to do with the operation of the engine. the tappet blocks are held in place by the barrels and does not need the case support there. the Triumph factory even eliminated the stitch bolts that used to be there on the T140's.
In my opinion you should be ok to put it back together and run it the way it is.
Possibly the finest private collection of Whitworth screwdrivers in the U.S.