Nice! Quadrant introduced both mechanic-side-valves and magnetos for 1908, so your motor is almost certainly 1908 or earlier. Of course Quadrant were one of the real pioneer makers - in the 1920s they bragged that they were "Britain's oldest maker of motorcycles" (or similar) and frankly their bikes looked like it!
I know they made their own motors in 1903 (when they famously used just one lever to control everything), so maybe yours is in the range 1903-08. Photos might help.
Leon
