If that is the case, it needs a new trigger wheel, about £5, available from TRIGGERWHEELS.COM, you will have to ACCURATELY have this put on, but it is only a simple machining job, once pulley is off.
If you cannot get a 90degree's disc, just get any and cut out uneeeded ones, so you end up with four lugs 90degrees apart.
Alternatively this would be easy for a light machine shop to make, and most likely cheaper than a new/s/h one ?
tabetha