The mk1 gearboxes fitted to the early 1116cc and 1301cc engines are the same as those fitted to the mk1 999cc FIRE engines. The differences are the bellhousing and final drive. The later 70SX models used an updated Strada based gearbox.
I looked at fitting a 1301cc gearbox to my original mk1 999cc FIRE so I could convert from 4 to 5 speed. However, the bellhousing also locates the gearshafts so it's a lot more hassle than just swapping it over as you have to have the right gaskets and ensure that everything is lined up. I think you have to check for endfloat as well.
Similar gearboxes were fitted to the Fiat Strada/ Regatta and also the Lancia Delta 1.3/ 1.5 engines as these used the same basic engines as fitted to the Uno. Other possibilities are the Fiat 128 models though I'm not sure if these were the same gearboxes. However, all these models are very rare and you'll be very unlikely to find one in a scrap yard.
Your other option is to look at the mk2 Uno and Tipo gearboxes fitted to the 1.4/ 1.6 models. These are evolutions of the 1301cc Uno engine but the 'boxes are quite different (hydraulic actuation and different linkages for a start). If you were to find a 1.4 Uno you should be able to swap the gearbox over with all the associated parts.
Sadly, most of the 1116cc/ 1301cc Unos have been wrecked by grass track racers so they are now virtually extinct. I doubt if you'll find any to take a gearbox from. In fact I've seen only three mk1 70SX's turn up on Ebay within the last year, one of which is now mine :slayer: