I can supply engine plates for pre 1962 frames, and also for the TE/TES type frame with the continuous top-tube which bends down behind the engine and gear box in one piece.
Engine plates are usually produced in 3 mm steel but stainless and aluminium can also be supplied. Please contact me to ask for technical information relevant to building the Triumph unit 350/500 into a Greeves frame – or see my Ebay listing for the information fact file that I sell.