Here's something that may help - unscrew the glowplug about 1/4turn, enough to release some of the compression, that'll take some stress off the pull start. Soon as it fires, use igniter to snug plug back down. Soon as igniter is removed use wrench to tighten glow plug all the way.

This is a common trick used to help high compression, tight engines - especially using a starter box.

If you are repairing the pull start rope, upgrade to the biggest diameter nylon rope you can run.

Good luck!