They are availabe pm more-torque on planetsand here is a post made by him.
The casting is the exact same casting as the original cubs. You may have noticed that most cubs are designed as 65mm bore cylinders. You can easily see that the 68mm bore cylinders have slightly different port heights than the 65mm. Actually, all of the ports change slightly in relationship to each other as the bore gets bigger on a cylinder because they have different angles as they meet the bore. So the Supercub does have some incidental differences that are meerly the result of the larger bore. But the casting is the same. It is only machined differently. Heads are in stock now, and all the parts are available to put together 4mm to 14mm Supercubs.