It seems the magnet does his work ,the belt is too long .
The new belt is 15.7cm yours is 17,8 cm probably the motor runs too fast because the slip from the worn belt is not enough .
The speed from the motor depents from the voltage,freq.and the load !
First try a new belt from Schopper ,please not a replacement !
If you can"t get it send me a pm.
The Thorens motorsupply is a 200 Volt with his own analog50 hz oscillator with very low distorsion and powerstage .
The outputvoltage and freq.can be adjusted and there is a analogvoltmeter on the front.it can be used for 124,135,134 Garrard 301,401,Lenco L75
Best is that when you use this supply you will be surprised that the sound is so much better then without it .
Everthing has to do with the pollution from the mains, I have done measurements where you can see all those noise etc.
But thats another topic.