I have a reproduction I created using Photoshop so the process (see pics below) would be the same as the Super Pro Football restoration, where I splice in the missing sections. This time I want to line the cardboard up so that the seams are smooth. It will be a bit tricky due to the reproduction being on slightly thinner cardstock but it should be possible. Just lining up the graphics/text will be the hard part. It also appears that whatever cart retention was there originally was destroyed or gutted so that will have to be replaced. I'm thinking it may be better to source a donor retention piece from a common box but I could also recreate one. Depends on what the next owner wants. I can also convert it to a top retention, Coleco box style and add a manual pocket as I did for my reproductions in the past.
Repro front:
Repro back:
Top retention and manual pocket (on Super Pro football repair):