The motor in my inQT-15SP (year 1988) should be the same SJ type like on other QT's from that time. First of all You should disconnect motor phase wires and somehow mark them (phases U, W, V) to avoid any connection mistakes later, then disconnect connector from magnesensor board and fan wires. Remove motor out of the machine (engine crane or somewhat similar would be very useful)- I just slip it out on a wooden planks.
Then carefully remove magnesensor and its disc (located underneath aluminium cover)- for that purpose there is two threaded holes in it (as far as I remember for M8 screws).Under that disc is located 4 screws that hold bearing in motor aluminium cover with round plate. I replaced both bearings in motor (this is a recommended practice in these cases)- in front side where pulley is mounted sits 6310N bearing. In rear where we removed that magnesensor disc is located 6308N bearing. Now here the most complex part is removing of pulley to get to 6310N bearing (both bearings and pulley are pressed on rotor shaft). You can use any suitable pulley remover to take these off or use a press. Replace bearings and assemble everything together. Only notable thing here is setting of magnesensor- it looks like aluminium cylinder that is fixed at a certain distance from magnesensor disc. Distance should be set at 0.25mm- for that just place a thickness gauge blade between sensor and disc- and fix sensor by two screws. Thats all. Reconnection of electrical wires is the easiest task here.