After some careful measuring, it looks like the TJ Wrangler flywheel doesn't fit well in the KJ bellhousing. I would either need to machine the flywheel down about 6mm to reduce the diameter, or use a 4.0L bellhousing. I decided that to keep the parts as stock as possible, a new bellhousing was the way to go, and Brett Quodling from Jeep City came through once again.
I now need an adapter plate to physically mount the motor to the gearbox, and a coupler to interface the motor shaft to the flywheel. Canadian EV is usually a good source adapter plates and couplers, but the first step is some more precise measurements. Unfortuntely while Canadian EV do sell an adapter plate for a 4.0L Jeep motor, my Kostov motor does not have the same bolt pattern as the Netgain motors for which they are designed. The Kostov mounting bolts are 8 bolts on a 252mm diameter 45 degrees apart, and the Netgain mounting bolts are 4 bolts on a 8.4" (213.36mm) diameter 90 degrees apart. It looks like the motor shaft is also different where the Kostov has a motor shaft of 35.5mm diameter and 40mm length, and the Netgain has a shaft of 1.125" (28.575mm) diameter and 1.5" (38.09mm) length.