Hard to say if anything else is wrong. Those check valves can be expensive but you may be able to get a good used one from a tractor recycler. I believe there is also someone out there who rebuilds them but below is a link to a video on how to rebuild them. They are not that complicated. I had trouble with one of them on a JD 317 and I was able to get the valve moving again by lubricating the piston and cleaning the corrosion off of the top part of it. I didn’t need to take it apart.
You also might want to clean the hydro after you get the leak fixed. It may be running hot due to lack of cooling from the fins and the body due to the coating of oil and dirt etc.