But the starter is "whirring" which means the solenoid is at
least partially working, no solenoid no "whirr" (highly technical term there
) The solenoid is there to handle the several hundred amps that a starter draws, and closes a pair of pad contacts very very quickly, a normal mechanical switch would burn out after a few activations as it cannot act quickly enough and would draw an arc, and burn the contacts out, even solenoids give up after a while and this ones probably 30 years old if the solenoids internal contact pads are burnt then it cannot pass the required current and thus the starter does not have the required torque to engage the ring and drive it
Also Its not unusual to have a motors windings go faulty and thus it only "whirrs" a little bit, not enough to engage the ring and drive it, I have also seen the rotor end bush collapse and prevent the rotor from turning freely
Stop faffing around with worn out bits and bobs
Buy a reconditioned one, theres plenty about.....they also whirr better
A new gear reduction starter can be had for less than £150 quid if you know where to look