Brenden Metz of Killa Bee's Robotics put together a write up on turning a $12 16mm planetary gearbox into a 16mm brushless ant weight drive.

How to build 16mm all metal gear brushless planetary gear motors.

Motors I used are the Xing 1104 4200kv Brushless motors. These can be hard to find at at times and I suspect any 11xx or 12xx with a 1.5mm shaft will work but the adapter plate will have to be changed.

The gearboxes are ChiHai 16mm planetary gearboxes. You can get them from AliExpress for about $12-13 plus shipping. Shipping takes a while, so plan ahead.  I use the 23:1 ratio.

Available here: ChiHai 16mm planetary gearbox

ChiHai 16mm motor dimensions (with original 050 brushed motor)
Comparison to the 16mm spur gear
23 grams as I built it
The original motor
Remove the motor casing. Caleb from Kill More Robots has teardown video: https://www.youtube.com/watch?v=ZpJ-m4tvtPo
pinion sticks out 0.072" (1.85mm) from Mounting plate
Remove the cir-clip, pull the brushless motor apart and unpress the shaft.
The Pinion sticks out 0.255" from base of motor. Make sure your pinion is sticking out 0.255" from top of the adapter plate
Drawing of adapter plate that I made. I used 2mm aluminum. I would recommend 1.5mm Aluminum. Now all you need to do is make a Dxf like this and SEND it out to a company that will CUT it for you and SEND back the completed pieces. (http://sendcutsend.com)
Tapped and countersunk adapter plates.
Adapter plate screwed onto brushless stator
After removing the gear box by unscrewing the 6 screws holding it on. you will need to remove the shaft and pinion,
At first I could not remove the whole shaft and pinion together so I took off the pinion and repressed it afterward.
Shown removed with pinion and shaft together. I press it out as far as I can then cut off the shaft with a Dremel.
Press the new shaft/pinion into the brushless motor
Shown pre-pressed. Press down to 0.072"
Put everything back together and you are done you Busy little Bee.
I use multistar 21A with bl_heli32