RE: OS. Mod
i am sure if i can get the LA to spin a 9x* at 16,500-17,000 i can get it to spin a 8x6 at 19,000. you can take the stock carb and open them up pretty easily. many mods can be done. we have taken small carb engines designed a venturi for them. then milled a 1/16-1/8" grove on the inside of the case where the carb rests. dril 3-5 holes in venturi where the slot is and run a remote needle to get the hole big enough. need the carb to match the crank hole size.
If have a crank that can not be opened up then shorten the length of the crank runner.
Also sometimes bigger isnt better. you can have a high velocity that will carry more momentum which can pack more fuel air in. it is all a balancing act.
Biggest thing when experimenting is to make one change at a time then run it and then do another. when you no longer see an increase then stop. also if grinding ports go slowly and very little at a time. for the exhaust ports i have found that for most engines have the top of the port wider than the bottom. then put a taper on the port upwards 15-20degree angle.