When I was looking the other year for a 2nd hand gearbox -fifth had cooked itself when the selector oil seal had gone, cheaper to buy 2nd hand than rebuild -I found out that all gearboxes fitted in the SDI from 2001 until the end of production were FFV. With ETF being a factory replacement?
But both the Arosa and the Lupo SDI were all uniformly AKU engined, so theoretically an earlier Arosa gearbox should fit fine.
Also, about the oil seal, replace that before refitting the gearbox!