I can't think of any reason why you would want to centralise the spark plug myself.
The plug being so central restricts the valve size possible and I though the original reason why Alfa (Autodelta) went twin plug ... 2 enable larger valves (with the original 90 degree head).
In the end as vintage prep said the flatter combustion chamber of the latter GTAm heads and then the modern twin spark ones has to be the biggest advantage.
While we all spend a fortune worrying about valve sizes, etc. a simple Alfa Sud engine with only 2 valves in a dead flat, non central spark plug will flow enough air to make 200 hp out of 1600cc at 8500 to 9000 rpm ... and yeah I have not the faintest about emissions but she was very good right through the rev range (except maybe below 3000 rpm) and had bags of torque.
I have also seen (on the same dyno) a 1962cc Alfa Nord engine make 208 hp without moving the plug or fancy bottom end, etc. other than baffling/windage tray. Now as that engine was not mine I am not 100% on it being reliable or not. I do not even know if it had fancy rods, etc.
But it is all good fun
, and reading all these sorts of threads makes me want to go racing again
I must say if I was going to race a Alfa Nord engine, the first thing I would investigate is increasing the bore so she's like 1999cc ... why give away capacity? and the bore is too small anyway, and has anybody tried to change the valve angle of a 90 degree head a little. With the larger bore you might be able to flatten the valve angle by as much as 5 degrees (assuming the cams might be able to run slightly closer together too). Now this would be massive work but we do know that flatter combustion chambers mean lighter and flatter pistons (thus more of the bang pushes down instead of sideways) and enable steeper inlet ports, etc.
Race preparing any engine in a lot of cases means work against design restrictions/limitations made by the manufacturers ... unfortunately the Alfa Nord engine being such an old design has plenty of really big limitations. It is amazing and a credit to all who develop these motors that they do so well on the track ... maybe these design limitations are overstated?. Then I get back to my little simple Sud engine which goes against ideal combustion chamber theories ...