Since it is a 4 jaw, it would be possible to make replacement, using the 3 jaw.
No chance of being able to obtain a new replacement?
Someone might even have new old stock one.
Maybe giving dimensions, you might find another make of chuck where a new screw would fit; not easy, but if you don’t ask, you don’t get.
But, if you are a newbie, this will be difficult for you, until you have more experience.
Whereabouts are you?
Maybe, someone close by could help; and you would gain a lot experience just by watching, or being guided to do the work.
If this could be done you would be up and running again, gain experience, confidence, and have the satisfaction of having made the repair.
Good Luck!
Howard