Are you sure it’s not just falling out of the powerband? What RPMs is it faltering at? It’s normal for these motors to feel like someone cut the ignition at lower RPM, they dont make anything until 9000ish. You can kinda guess your main jetting by the top RPM that the motor stops pulling at, but that actual RPM depends on your setup.
Run 24:1. Do not pass go, do not collect $200. Nobody is jetting these on 32:1, so get that variable out of the way.
Change nothing else until you are running with that. Many 80’s are fairly highly strung so proceed with caution when lowering octane. To be honest, stick with a leaded racing fuel if you can. 91, even with stock ports and head is going to be sketchy if you have a kart racing pipe on it. Lead is verykind to the bearing surfaces in the motor. Run a racing oil too, not some random road motorcycle stuff. Motul 2T is a popular synth, Maxima 927 is a popular castor.
If you have a pumparound carb, ensure both sides of the fuel pump are clean. A weak return side will cause it to run rich.
B10 is fine, again those motors are small but tend to have high specific output.