Anyone know anything about converting a goped?

I have an old goped liquimatic im using for a science fair project, but the current spindle is heavily worn and causing it to not run as good. I called a parts dealer and they said i could replace it with a Sport model spindle, but wasnt clear in the directions. I already have the spindle that was removed, but the new one wont slide over the shaft(its not the right one for that spindle), so do i have to replace the shaft too or just remove part of it. Any help is great.

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