Which wires are being jumped? The solenoid on the fender near the battery or the terminals on the starter itself?
The relay on the inner fender can be operated by connecting the small terminal to the 'hot' side connector. That simulates key switch action and if successful, points to the ignition switch itself or safety{clutch pedal/auto neutral} switch preventing power to operate the relay being the source of the problem.
If jumpering the relay just causes clicking or buzzing, then more testing is required.
If jumpering around the relay using LARGE jumper cables enables the starter, then the relay itself may have worn contacts and need repair or replacement.
If jumpering past the relay results in clicking, then the solenoid on/in the starter or the starter itself may be failing. That's when you try the hammer test [gently, as things can and will break when hammers are brought into play. Some starter motors have ceramic magnets that can shatter, and then it is new starter time]
tom