That is where the wires from the pick up coil and the stator leave the crankcase. There is no part number for the seal, it is part of the stator/pick up coil assembly.
Depending upon the model year of your bike, you can buy the coil and stator separately. Later models have the coil and stator as a unit, and will cost you about $250.00
Replacing the coil/stator:
Site to download the service manual for the 650: