The pin is approximately 3/32" in diameter. Any good hardware store should be able to provide you with a roll pin or steel material of the right diameter. A drill bit, welding rod, coat hanger, screw...something steel will suffice. If you remove the clutch actuator cover, left side of engine you'll find the cam portion of the actuator that presses on a steel pin in the middle of the drive sprocket. That cam is held in place, or should be, by the pin that is obviously missing in your engine. Some are peened in place, others are held in with a glob of epoxy material. Make your replacement long enough to be flush with the surface then secure it with either method mentioned above.
Good luck. Let us know if you need more help. This is a common problem with some Chinese 2 stroke engines.
Tom