the best thing you can do for these bikes to get more out of a stock engine is to look into the ignition imo. I do a lot of small block ford and chevy related builds and timing advance is everything. not sure what these are set to for initial timing, but they get pretty late after 5000 rpm.
they really need a progressive spark timing system.
also higher compression is always a way to gain more power for no money 0$. here is how I did it. it increased the torque by a noticeable amount!