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