You can remove the roller part of the cars with an allen key unscrewing both sides. You can see the “hole” in your photo. These are likely to be seized due to salt etc so you may find it easier to remove the car itself from the track so you can sit it such that a penetrant can soak in. You could also first try pouring boiling hot water on it (not while it is on the deck!).
Replacing the actual car is difficult because, as you have found Harken stopped making them (WHY!). I got various quotes for them from a number of suppliers but ended up with a company recommended to me via Ebay. He made custom anchor rollers but these were white; for a bit more he bought in some black delrin and made them to my spec. They have worked well since.
There is an excellent (well I wrote it!) article in Dufour News 121.