I don't know if there is a repair person or a Forum member near you that could help you in person.
There are two likely reasons the Quill Return Spring is not working properly. The first is as you suggested, it is broken. The second could be the tension is not set properly. This is fairly easy to check. On the top of the Headstock above the Quill is a set screw which if it has never been removed is covered with putty. If the tension is the problem the putty has likely been removed previously. Remove the putty and then the set screw. Do not lose this set screw since it is not a common cup end set screw and harder to replace. This will allow the Quill to be removed from the Headstock. With the Quill out continue to turn the Quill Lever in the same direction to see if you get any tension. If so, that is the problem. Slowly release the tension to neutral. Since you have the Quill out it is a good time for maintenance. Clean the Quill sleeve and wax it per the maintenance suggestion in the Owner's Guide. When you are ready to put the Quill back into the Headstock turn the Quill Lever to get tension with 3 or 4 revolutions than insert the Quill and retract it. Put the set screw back in the top of the Headstock. The dog end of that set screw goes into the groove on the top of the Quill and keeps it from rotating and coming completely out of the Headstock. Do not completely lock down that set screw. Check the operation of the Quill extending and retracting.
If the Quill Return Spring is broken that is not a difficult fix either. It requires removing the Quill Feed Pinion Assembly. There is a set screw, again covered with putty if it has never been removed over the Spring Housing above the Quill Feed Pinion Assembly on the back side of the Headstock. If that set screw is loose it could be the cause of the lack of tension. Removing that set screw and the Quill Lock & washers from the front end of the Quill Feed Assembly will allow you to slide the Quill Feed Assembly out the back side of the Headstock. Removing the Feed Stop Lock, Depth Gauge and the Washers will allow you to remove the broken Spring in its housing. Keep all the Washers and other parts in order to reinstall them correctly during reassembly. The end of the spring hooks on a pin to the Quill Feed Pinion. With a replacement Spring and housing reverse the order to install it. Don't forget to re-tension the spring as previously described by removing the Quill.
Watch the My Growth Rings video Scott linked in his answer to your other post. He shows removing the Quill Feed Pinion Assembly and re-tensioning the spring.
A replacement can be purchased from Shopsmith and can be found here on the website under Products, Service, Service Parts, Mark 7/Mark V Models/ Newer Mark Models, Headstock. It is part number 521875 Quill Housing w/Spring and is $33.37 plus shipping. The spring is only sold with it already installed in the housing. A link to the service page for the conventional Headstock is below.
https://www.shopsmith.com/service/headstock/index.htm