Follow up, I fixed the pull start on the mower though probably not as good as before. Turns out rewinding the spring was the easy part.
Threading the cord through the T shape pull handle on the other hand. I got one of those 'easy start' ones that is mounted to the the frame which was the reason it frayed in the first place because it got caught under something else
What a ****, it took ages. It took fine tuning between melting the cut and making the hole bigger. I hate that.
