Had to get up "early" for a Swap Meet.
The 2 hour drive gave me time to consider driveline options.
Option 1 - Chain drive
On the twin engine dragsters, back in the 60s,
one method coupled the engines front to back using a sprocket on each motor, and a double row chain to connect the two together.
Pretty simple, gets away from a few problems.
I think you could use the same idea, attach a sprocket to the back of the motor, and a matching sprocket to the front of the Japanese bevel drive. Add short piece of double row chain, and a few mounting brackets.
The output from the bevel drive(left side) might need a simple extension, with a sprocket, to drive a Harley transmission.
Then a regular chain drive to the rear wheel.
Option 2 - Shaft drive
Turn the HD trans 90 degrees, so the input shaft is on the front.
Couple the clutch basket to the motor, using the double row chain method. Chain output from the trans runs to a jackshaft on one side, the shaft drive attaches to the end of the jackshaft.
I think the first idea is easier, and chain drive
gives you more options for wheels etc..