3) Wheels A. BSA wide wheels are the only wheels allowed B. 1.170" Minimum Diameter/ 7.50mm tread width (Tread width will be measured from the outside of the decorative edge to the inside edge of the wheel) C. Wheels may be lathe turned, polished and/or sanded to true the tread surface, diameter, and the edge of the tread. D. The tread surface must be flat and parallel to the wheel bore E. Removal of the inner and outer step is allowed as well as re-coning of the inner hub. F. Wheel Bore treatment is allowed including polishing and/or tapping. G. You may add material to the inside of the wheel to aid in balancing H. Wheel covers may be used only if the entire wheel is visible I. The following wheel modifications are prohibited: 1. Drilling holes in the sidewalls or tread area 2. Grooving, H-cutting or V-cutting 3. Altering the wheel profile 4. Filling of any wheel surface with any type of material 5. Removing or modifying plastic material from inside the wheel. 6. Removal of BSA markings and spokes
You are allowed to add material to aid in balancing the wheel, but you are not allowed to fill the inside of the wheel. I would say that if you filled the inside of the wheel with any material that it would only slow you down. I have filled the inside of the wheels in testing to see if there was any benefit to doing so and I could not find any.
You are allowed to tap the wheel bores, but you are not allowed to fill and drill new bores.