Novak's Nitro to Brushless Conversion Kits

[Serum]

This sounds very promising;

6S, 5mm shaft, wider range of KV.

Now the next step is to upgrade the rotor to a slightly larger diameter to increase the torque, to enable the use the full potential of 6S; i think that because of the smaller magnets, you will hit the limits of a motor sooner. (copper and iron losses)