May be you can control the manganese level in your GO by modifying the wash of the material, since permanganate is a synthetic reactive in the hummers method.
You may use hydrothermal or solvothermal process for the doping process of Mn to prepare Mn-doped GO. However, the doping of the metallic Mn may to tricky or problematic as you have to use Mn-salts (in various oxidation state). Thus, an in-situ reducing environment is also needed to convert various Mn-ions to Mn(0) state onto the GO surface. I can say thsi from my knowledge.
Wish you all the best and waiting for the other RG experts opinions.