Zorro,
When you rebuilt the motor, did you fit the camwheels on the correct side? They are marked up 'L' and 'R', but that is as viewed from the front of the motor, so the wheel marked 'L' is actually on the right hand side of the machine, and vise versa. Whilst the gears look identical, one has the keyway aligned with a tooth and the other aligns with the space between two teeth. If the wheels have been swapped over, the valve timing would be far enough out to cause one side to run hot.
Regards,
Eddie.