The gears are available now on Tindie.

With the sale, I can partly finance my hobby: inventing and publishing it. The further development of my Maxun One solar bike is expensive, many parts are unique and are manufactured by companies to my specifications.

See the article here.

As with the Yamaha PW-SE, the nylon gear is a weak part of the motor. When the motor is broken, it will just be exchanged by Yamaha. Outside the warranty period, this will cost around $750. This is a big problem because, when the gear will be overloaded again, you can keep going this way.
The sprocket is milled from PA6 which is a very strong material, but I cannot have it injection molded because the numbers are too small, which unfortunately makes production expensive.

Limit extreme stress to prevent a sprocket wear again

The application of a nylon gearwheel at this place in the motor is definitely a design flaw as the gearwheel apparently cannot withstand the maximum motor torque as it breaks too often. In order to prevent a failure again, you will have to avoid or reduce extreme loads from now on. An extreme load is driving at full throttle at low speeds because that is when the motor torque is greatest. The best solution would be a software update from Yamaha where the motor torque is automatically kept below a safe value. 

Know that nylon gearwheels are extremely strong when used properly: they are used successfully in front wheel hub motors and rarely break.

Replacement

Remove the broken gear and glue the new one in place. Use common one or two component PU polyurethane adhesive. PU glue has been tested in practice and is very durable. Here polyurethane glue from Bison, note that the text may differ per country, as long as it is polyurethane then it is fine.

Lubrication

To lubricate the gear wheel, you can apply the excess grease that can be found anywhere in the motor onto the gear wheel.