Talk to the FAA, who have made it so expensive to upgrade/update anything related to airplanes. There is a marginally viable aftermarket parts market for these engines now, but for a long time owners were strictly limited to OEM, salvage, and remanufactured parts. Also, the number of older GA Airplanes, and antique/warbirds in service are so small, it's got to be nothing compared to the exposure from thirty years ago.