In the early 1960s, my little brother built a "racing" Bantam for use on the road, George Todd cylinder head, 3-speed close ratio gears, total loss ignition (flywheel removed, using points and battery), Noton fuel tank, clip-ons, etc. Great fun to ride (once you got above 30mph in first gear, using the clutch), used to give my 1934 KSS a good run.
I was the local village Guru for setting up Arrows (and Leaders) for imroved road performance.
The "Golden" Arrow had a 1.125" carb as opposed to the standard 0.875" so that was the first thing changed. The later models performed better than the earlier ones because that had the squish-band heads with the central plugs - another "must". These heads responded to the usual skimming. All this, plus the usual fine port polishing and precise ignition timing, gave these little bikes a respectable peformance.
I can't remember the finished compression ratios but then it was measured geometrically from the stroke, rather than from when the exhaust port was closed, as they do today. As I worked in a metallurgical laboratory at the time, much ahem, works time was spent on measuring the cylinder head capacities, with a burette, after skimming and polishing, to ensure balancing of the compressions.
Oh well happy days (daze).