Ah, missed that.
So the lead going to the injector, it looks like you will have to "break" it to get it through the hole. I would say wrap that injector lead around the sensor as many times as you can stand it, the more it looks like this, the stronger the magnetic signal to the hall sensor should be when current is flowing through the injector:
Or just wrap some appropriately thick magnet wire around the sensor and put it in series with the injector lead.