First comment. I am interested at how you arrive at a CR of 11:1
It certainly beats anything I have ever achieved.
If I fill the head volume with thick oil with piston at TDC, I have 23cc
On counter bored plug pocket type i.e. Mk1 head patts as used on racer up to 2004
Volume at TDC 23cc
If calc approx from half stroke i.e. 320/2 160cc into 23cc then theoretical CR is 6.9:1
This assumes 100% filling which is not reasonable to expect, given the nature and direction of the inlet tract.
I put reed valves on the doors of a Scott in 1967 and the cylinder filling was then much more complete and you had to jump on the kick starter to get it to turn over.
Next. If the noise was from pre ignition, then I would expect it to vary with the ignition advance. I would also expect that the plugs would show signs of distress unless they were far too soft.
I would be looking for a mechanical source.
Not likely, but worth checking, given the damage that it can cause, is to be sure that the crank tapers are snugly home.
Get hold of the flywheel rim and try and wag it.
When the engine runs, does the visual appearance of the flywheel stay reasonably crisp, or does it go into a foggy blur?
Check the nut that holds the clutch on to the gearbox high gear. I have known these to strip threads and the whole clutch be clattering loose.
In this case it would be stabilised when the clutch was pulled in and the clutch backplate loaded against the head of the big bronze bush that the high gear runs in.
We do assume that the rivets that hold the 20T primary drive sprocket on the flywheel have not become loose. They are only 1/4″ mild steel pins in clearance holes and we often see quite spectacular wear here.
Just a point that can catch us all out if we are not careful. If you make a new key for the flywheel to crank location, care should be taken to be sure that the key is not too tall, or too long. If it is too tall, you can assemble an engine where instead of the cranks sitting snugly in the flywheel tapers, they are sitting on top of the key and in line contact only on the opposite side of the taper. If too long, the cranks can bottom on the key and not in the tapers. Its worth a check!
Otherwise if you have big oversize pistons fitted, is there clearance for the skirts in the case. The smallest obstruction will make quite a big noise. A great opportunity to prove your dogged nature?
Please excuse if some of the above is a little simplistic, but after so many years trouble shooting special machine tools, I came to the conclusion that you have to start at the bottom and hope folks will not imagine that you are trying to insult their intelligence.