I'm assuming the 350 is the same as the 250 mono. The tool was thick sheet metal with a spanner on one end which fitted, I think, the spark plug. On the other end was a short piece of a large allen key which was used to take off the front wheel, it fitted into a socket in the axle. As far as I can remember it was a 10mm allen key, but I wouldn't bet on it, I sold my last mono ten years ago.