I bought now (March 2010) new tyres from Munich (www.oldtimer-reifen.de).

There is a Radial tyre made by Michelin. I choose the radial type instead of the diagonal type for security and comfort, the price was nearly the same as for the diagonal type.

Michelin X-Stop

5.50×16 X-Stop

Marke:Michelin
Dimension:5.50R16 H(84) (175/80R16)
Tubetype/
Tubeless:
Tubetype
Tragfähigkeit:500 kg
Zul. Höchstgeschw.:210 km/h
Max. Breite:175 mm
Max. Höhe:685 mm

The correct tube for the A90 Atlantic convertible rim is the TR-15 one.

Tube TR-15

The TR-13 also fits, because its smaller, but to avoid movement of the tube, the TR-15 should be taken.

Tube TR-13
Recommended Tyre Pressure:    
 2 Passengers Full loaaded 
 FrontRearFrontRear
Atlantic26 lbs./inch² 1,79 bar26 lbs./inch² 1,79 bar26 lbs./inch² 1,79 bar28 lbs./inch² 1,93 bar