I don't think moving the negative wire to the starter would be the problem.
Maybe first be sure the battery and charging system is OK. You will need a volt meter. The charged battery should be about 12.7 volts DC, engine not running. Start the motor, check battery voltage again maybe rev it up just a little. Give it a minute then check battery vlotage again. It should be about 14 Volts DC if the battery is fully charged. It could take it a minute to come up. If the voltage does not come up you are not charging.
The starter is the highest current device on the bike so lots of current. It does not take much resistance to be a problem. Fortunitly there is not much in this circuit. Maybe disconnect the negative terminal frome the battery for safty and clean all the connections in either positive or negative wires, terminals, bolts, etc. losen the connection clean it and reconnect it one at a time. There is a starter relay, remove it and look at it, try to see if the contacts are carboned up or burned.
For a test you could try connecting the volt meter leads between the batttery terminal and the starter. Crank the starter if you read a voltage that would be a drop somewhere in the wireing. Do this with both positive and negative if you can. This voltage should be very low you do not want much drop in the wires.
If it passes all these and the starter is OK I would think the problem is mechanical. Maybe the starter clutch. Perhaps by then some one with some mechanical experience will come along.