It is not for the rip cord, for the entire pull start, if you have never rewound one before, it is not worth it to try to get just the spring and I don't think they sell the spring itself, even if they do, it will be as much as the entire pull start. Once you have the dimensions, you can order...
Now you need to find out the size of the backplate you need. That is the distance between 2 adjacent holes (center to center) and then you can get the right size.
Here is some information I found about size of blackplate and hex nut size. There are more but they are for older O.S. and Kyosho engines.
100% Brand New
Rear Cover: 11011 & 11012
11011 rear cover
Applicable engine: VX SH 16 18 21 grade
Specifications: Hole distance 27MM, Hex size 12MM Or...
When you separate the pull start from the engine, does the pull start spring back. But if it doesn't has a spring in it, obviously, it won't spring back. I think you can get an HSP pull start with the 14mm hex size and check the size of the back plate as well, there are 2 sizes. Good luck. If...