Rotax 912 A Gearbox (Old Version) Eng S/N 3792814

Have you contacted the Rotax distributor that is in charge of your area?

http://www.cfsaero.com/

Yes we have.

With no result :-(

The case and the case adapter are different, but most of the "A" box internal components (bearings, seals, etc.) are the same as the "B" gearbox. So you might want to look into sourcing "B" gearbox parts... (Stock is going to be very limited for "B" gearbox as well, so you may need to look farther afield than your local distributor.)