We've done four RR6-150 conversions on our 250 2Cs so far and I imagine it would be a similar proposal to the 125. Honestly, it's not just a "plug into the loom" job. The AC feeds from the generator coil have to be disconnected (one is soldered to the coil body) and isolated from earth (I just use shrink tube). Your existing regulator/rectifier will need to be removed and this will probably entail some major surgery to the loom.
In all cases we've completely rewired the bikes. It's not a big job (really!) and the electrics are transformed with the RR6-150. Steady 6 volts DC and excellent battery charging.
Happy to assist if you contact me through the web page
Benelli 2C Restoration