Steve,

a 50cc engine that can't turn a 20" prop beyond 6100 is not a strong engine. IMO, the performance you are looking for will start with a stronger engine. DL, Brillelli, FPE, 3W , DA, are va few companies that have offerings in that size range, that would give you better performance on a 22 X 8 to 23 X 10 prop. If you can't sell that lead sled of an engine and upgrade, perhaps dropping to a 20 X 8 Menz or Bolly, would get it more into the power RPM range.

Having said all that, soulds like your Bipe (very nice looking BTW) is flying pretty scale like as it is anyway.