Emerald Night Piston replacement + jamming issue
There is a Bricklink store that sells custom-made train driver rods. I've never bought any so I can't speak to the quality. There are supposedly limitless varieties possible but given that you're looking for one that matches a set you should be able to find some already-made for that set.



Regarding the jamming issue, the most common problem is when the wheels aren't properly quartered. When building, the pin holes for the pistons on one side should be at 12:00 (top) and on the other side at 3:00 (front). This helps prevent jamming.