Hi,
I'm doing this from memory of the last time I had my 50 Sport and 90 deLuxe apart a few years ago, But I don't remember it as being a big problem The hook on the inside end of the spring goes into a hole in the crankcase wall (at about the 7 o-clock position, I think), and you then need to grab the straight tang on the outer end of the spring, and turn it anti-clockwise by about half a turn and slot the end into the next hole in the kickstart shaft. (you might need strong wrists for this, or else a pair of pliers). Don't slot the spring end into the nearest hole in the shaft, otherwise there's not enough tension to return the lever. Then pop the aluminium slug into the centre of the spring and the job's done.
I'm assuming that no-one has dismantled the kickstart shaft itself. The end stop piece at the inner end of the shaft is press fitted onto splines on the shaft. There's no need to split this apart normally, but if someone has done so, it is possible to reassemble it on the wrong spline so that the return spring holes in the shaft are not in the right position relative to the stop piece.
Hopefully, though you won't need to check that far into your engine. Good luck.
Graham