You guys answered it all almost!
The kb10 is at 66hp its only 2hp off the a12 as a single carb setup.
An A12gx vs A10gx might be interesting both with oval port heads! :)

Although 10kg doesnt sound like much when you compare power to weight
of the single carb A10 from a KB10 vs the A12 then the A10 might win.

I personally prefer the A12 w/ 3.9 in my 1000 by along shot though :)
However my coupe with the A10 is a different experience than the VB10
its feels more sporty as you have to revv it more so requires a diff
driving style yet economy is incredible probably another 20% better.

A12 with 76mm bores cant go wrong though!

Id be more interested in comparing the most rare of all the A series egines

The A13 long and short stroke versions if they exist from past threads

eg
1289cc 77mm stroke and 73mm bores
vs
1270cc 70mm stroke and 76mm bores