My name is Geoff Clatworthy. I have built (with the assistance of others), a number of Australian ES2 racers now known as ES1, 2,3 and 4. The photos's elsewhere on these posts are of "ES3" (long stroke) with the obviously welded/modified inlet manifold, and "ES4" short stroke, both with the cross over gearchange linkage through the 3/4" s/a pivot bolt.
No quick shift other than the quality set up of the 5 speed TT Industries boxes and Bob Newby clutches (and rider skill).
Happy to answer not too long questions should I be able to assist. I have spent 40 years developing these bikes and don't believe that there is a lot more to be gained. I standard ES2 is claimed to have produced 28bhp (rear wheel?), where my modified iron ES1 produced 46bhp, ES2 and 3 between 49 and 51bhp and ES4 considerably more. They weigh about 238lb with fairings and oil without fuel; with performance reflecting the power to weight advantage over a Manx and some excelelnt riders Chris Anderson, Mike Soderland and Ian Lovell.
I believe the 4 valve ES2 to be an exercise constructed for the amusement of a clever English enginner utilising some speedway components and not intended for racing.
Kind regards from OZ - Clappers.