It is all a hassle because some parts, especially transmission, are hard to find now.
The best way to get parts is buying whole cars on ebay, but then your are left with lots of extra parts.
If you really want the slipper, here is all the parts you need.
A-3120 Slipper Shaft
A-3004 Gear Cover (old one doesn't have room)
A-6905 3/16" x 5/16 Bearing goes in spur
A-3121 Backing Plate
A-3122 Gear Plate
A-3123 Friction Pad
A-3124 Springs, Cup, Spacer and Washer
A-3125 9/16" x 1/4" Thrust Bearing Assy
A-6305 4-40 Locknut
A-3904 Slipper Gear 84T
A-3906 Slipper Gear 86T
A-3908 Slipper Gear 88T
A-3910 Slipper Gear 90T
I don't know enough about cars like the XX and B3 to tell you what makes them so much better than the XXX and B4. I skipped all those when I was out of the hobby.