The actual piece that is broken is the link (plastic) that attaches to the plunger (metal).
Go online to Sterns site and search the owners manual for the part number which will help you search of no one chimes in with the correct part number.
You don't need a whole kit...just plunger and link to make you life easier. Like this....but I don't know if that's the correct part
https://www.marcospecialties.com/pinball-parts/515-6304-03
Edit: 5cent lesson - the metal plunger is pulled in by the magnetism when the coil is activated, this also pulls the link with it which is attached to the crank that makes the flipper go up