Jerald, Jack Docwiller used to rebuild them or may have a new one. G-squared made replacement aluminum air filled and adustable for feel and lengthe of pull. I bought the last two they had years ago , but they may have made another run since then. Also Joe Shiozaki at J.S. air cushion may have some help.
If your unit is not cracked it can be rebuilt if the end cap will come out. I have taken to placing a spring inside the inner chamber instead od replacing the O-ring and oil. They all eventually leak and make a mess. Let me know and I can talk you through it. Jeff