We have an Ericson 34 that had a similar problem. The mast track has two sections with a splice that is just below the first spreader. There is a machine screw on either side of the splice holding the track to the mast.
This spring we were unable to raise the main past the splice in the mast track. On investigation, I found the head of the lower machine screw had snapped off and the bottom segment of the track had wiggled slightly out of the mast, so no longer in alignment with the upper track segment.
I used a small nail to feel inside the hole where the screw had been and could tell that the shank of the machine screw was still embedded in the mast. Stainless steel screws into aluminum eventually corrode and I could see some white powder indicating that the screw and mast had corroded. There was no way that I would be able to remove the screw shank through the hole in the mast track.
As it happens, our next door neighbor at the marina has an Ericson 35 with a Kenyon mast, and I asked him if he ever had a problem raising his main. He said he has the problem every spring, and just goes up the mast and hammers the mast track segments back into place! Sometimes he has to do it during the sailing season. Both machine screws of his mast track are missing, so he's depending on a tight fit between the track and the mast to keep things aligned.
To solve this for our boat, we installed a Tides Marine SailTrack system. There's lots of information about this system on this forum. We don't want to get trapped with a main sail that I could not lower because the mast track had again popped out of alignment. Even if a track segment gets slightly loose, the SailTrack spans the splice and prevents misalignment.
In your case, I don't think that rust is the problem. This is a stainless steel machine screw into aluminum. Instead, corrosion is the problem, and, if the screw spins freely, the threads in the screw or the mast have corroded away.
There is no nut on the inside of the mast to hold the machine screw. This would be impossible to install even at the factory. The mast track splice is at least 15 feet from the end of the mast and there is no way to securely hold and position a small nut that far inside the mast. The machine screw for the mast track is held via threads cut into the aluminum mast.
You MIGHT be able to drill out the existing hole in the mast and tap larger threads for a bigger machine screw. This assumes that there is enough good aluminum that hasn't corroded for tapping new threads. You'll also need to drill a larger countersunk hole in the track for the larger head of the larger machine screw. If you do this, be sure to use Tef Gel to help prevent corrosion in the future.