Don't know where to start with this....
This gearset is a dicey proposition. Putting mix and match components like this into an early 128 or X19 trans is a tough task for the un-initiated.
Given the 4.08 final drive of the pinion shaft that is included (Fiat X19), the 128 style 1st gear, the advisement that it works for X19 or 128, and the final kicker of "just use your existing 2nd - 4th gears" both of which are just wrong.
The pinion shaft is a 5-Spd style, so the nose of the shaft is larger diameter, requiring an alternate pinion bearing (4Spd and 5-Spd pinion bearings are different). Most USA 128s came with the 3.76 Ring and Pinion, so find another ring gear while you're at it.
X19 and 128 ratios changed repeatedly in the 4-Spd and 5-spd world. Nothing dramatic, but they loved to change the tooth count, and more insidious, tooth pitch on the cluster or layshaft gears for no fundamental reason.
Only 2nd gear stayed the same across all years. 1st gear was the next most consistent. 3-5th changed every other year for no apparent reason either in tooth count, tooth pitch or both!
Next comes the shifter select components. I assume they are for FWD Ritmo, but again, assume. Lot of potential headache there.
Yes, there are some fundamentally nice pieces here. But buyer beware, this is most certainly not a drop-in solution, and you will
need to have your engineering / details hat on to make it work.
Bottom line, just find an FI Yugo 5-Spd box for your 128, it will save a lot of headache.
-M