Edit: the motor that comes with this kit doesn’t fit properly. The mount diameter is too large for the knockout that comes on the newer gx390’s. you can dremel out the opening to fit, but it’s a pain in the neck!
This is the one I just bought, and am installing as I write this:
https://www.amazon.com/gp/aw/d/B01CK2ZPCI/ref=yo_ii_img?ie=UTF8&psc=1
Couple things you will need in addition to the kit:
Two M8x1.25 bolts to mount the motor
Three M6x1.0 bolts to mount the control unit
An M6x1.0 Tap to thread the holes where the control unit mounts
You may also find a puller useful. The original flywheel was a real treat to pry off
