My knowledge about scooters is quite small. Of course my thoughts already went around that topic, but without many results. In the contrary to H. Fenske , he is now in the run of building one. If you have suggestions or questions, direct them to him.
He solved the sealing problem with oil seal rings which are good down to 80m. Now he is in search of a motor which
Right now he uses a model motor, which is to expensive (new €250, used €175). Furthermore he needs a small gearing which is able to transmit the power !!!
And some Links:
EXTREME Do It
Yourself Plans!!!