A couple of thoughts:
1. I highly recommend modifying the track so that you can remove the car easily by simply removing one bolt at the aft end of the track. See my picture below on this, my prior post, and/or as shown in the video link posted above.
2. Once the car is removed (so you don't have penetrating oil dousing your gelcoat etc) and you can remove the PVC runners, patiently apply penetrating oil (PB Blaster, Kroil, Mouse Milk or similar) to the recess / counter-bore into which the shaft seats. Do this for a period of days or even weeks. You also might try heating the entire car (with the plastic parts removed) in an oven. I probably wouldn't exceed 200 degrees F. Periodically grip the shaft with the padded jaws of Vice-Grip pliers and rotate, even if you don't perceive any movement this will assist the penetrating oil. Sooner or later, with patience, the shaft should loosen and be removable. If those measures don't work and you have access to dry ice, do the oven treatment then apply dry ice to the shaft only (thereby shrinking it dimensionally).
3. If you haven't already sourced the springs and/or runners, they are available from Euro Marine Trading (the USA Antal Dealer) USA Phone # 401 849-0060 , 62 Halsey Street, Unit M, Newport, RI 02840 USA
1. Runner Inserts are G30, 40A PVC Inserts 160 ml long (US $ 18.45 per set of two in 2017)
2. Stainless Steel Stand-Up Bracket Spring part number Y-X1041 (US $ 7.86 each in 2019)
I wish you luck,
Gary S. Silver
s/v Liahona Amel SM 2000 Hull # 335
On the hard in Puerto Del Rey Marina, Puerto Rico