The 12.7 is probably too big for a 26 (You may find transition issues) if not on a highly tuned engine like a Zenoah or other high end type.
And the 9.5 too small. Restricting RPM by 500 to 1000.
The 11.1 venturi walbro is probably the best allround size for a chinese 26.
My SPE came with a 9.5 but I believe others had a 11.1 venturi's.