You might want to check the cables from the battery to the solenoid and solenoid to the starter, it's not unknown for them to go bad after a long time on the bike due to internal corrosion. CXPHREAK's advise will check this issue, just make sure you use a jumper that's the same size or larger (lower AWG number). I'd also check the ground wire from the battery to the frame. Also don't forget the battery terminals as well.
I've had issues with DC motors before with the commutator being dirty and not getting full current to the windings. Check to make sure that the slots between the copper bars of the commutator aren't full of carbon or the bars are worn down to the insulation. You can use a firm plastic brush or a plastic knife to clean out the carbon of the slots. If you have access to an Amp Clamp (clamp on ammeter) you can check to see if you're getting full current through the motor. Power is Volts times Amps so even if you have Voltage the motor won't turn if there isn't enough Amperage.
I've had issues with DC motors before with the commutator being dirty and not getting full current to the windings. Check to make sure that the slots between the copper bars of the commutator aren't full of carbon or the bars are worn down to the insulation. You can use a firm plastic brush or a plastic knife to clean out the carbon of the slots. If you have access to an Amp Clamp (clamp on ammeter) you can check to see if you're getting full current through the motor. Power is Volts times Amps so even if you have Voltage the motor won't turn if there isn't enough Amperage.
