Take it from the beginning, at least electrically, with the head speed that you want to run. Multiply this by the gearing reduction to get a ballpark number for the motor kv and cell count you will need.
As an example, if you want your head speed to be 1500 rpm and the gearing is 10:1, then your motor will need to be turning at a minimum 15000 rpm for a given cell count. As this is a somewhat heavy heli especially after the conversion, you'll probably be running 8 or even 10s lipos. With 10s at 37 volts nominal, a motor with a 450kv rating would get you there with a little headroom.
You're probably looking at a 100 amp esc, which is probably somewhat overkill since this is scale, but better a little too much here than not enough.
Battery capacity of 7200mah, assuming approx. 1400 watts average (maybe less) to fly, should get you 8 1/2 to 9 minutes of light using the 80% rule. I run (2) 6s 3600mah packs in parallel on my Hirobo Lama conversion and get around 10 minutes.
There's always experimentation here, but once that baby flies under electric power, you won't be able to wipe the grin off of your face.
Just plug in your numbers using the examples I gave and let us know what you get.
BTW, that conversion set looks really nice.