HZMM Retractable Key Chain Key-Rings - Heavy Duty Key Holder Belt Clip with Multitool Carabiner, Retractable Keychain Key Holder-Rings/Carabiner, 25 Inch Steel Wire Cord

Not sure what scale you're building this at but I would probably repurpose or copy the recoil mechanism from something like this. Then I'd recess a magnet (or multiple magnets if necessary) into the reel and use a hall effect sensor mounted on the static part of the shell to detect the magnet as it passed. This would allow for quite precise control over the spinning part of the model via the pull cord - spinning speed could be proportional to the speed you pulled the cord, for example.