i seem to recall that the nearest universal belt size has a wider band to it and thus it wont stay on the capstan of the motor. if you can find a precise way of slicing it - let me know! any imperfection in the cut will create a weakness that wont last..
this is the reason why people have to buy from the aforementioned supplier. i think he must get them made or something..
there is a bloke on UK ebay that sells the various tape drive belts, maybe message him and see if he can supply or advise an alternative?