Sowerby heirs contacts

Sorry this topic is so narrow and long-winded! But I’m hoping someone here will know.

The American composer Leo Sowerby died on July 7, 1968. At first there were three ways of contacting his heirs: 1. via the Sowerby Foundation, run by his friend Francis Crociata. They had a newsletter, a website, etc. But Mr. Crociata has since passed away, the website has disappeared, and anything sent to their old address bounces back. 2. His executor was Ronald Stalford who, though not a relative himself, knew hot to contact the heirs. Mr. Stalford has since passed away and I have been unable to find out who took his place as executor. 3. The Washington National Cathedral, where Dr. Sowerby worked for the last several years of his life, used to know how to contact the heirs, but has lost touch with them.

My problem is that I need permission from the heirs or estate to copy a handwritten manuscript on file at the Library of Congress. They’ll let me look at it, but that’s it, and I need to have a copy for some research I’m doing.

Does anybody on this forum have the remotest clue what I should do next to solve this? Thanks in advance for any help!