That is a common problem. Perhaps you can find a new seal for the mc lid you have?
Clean up the top surface of the MC casting too.
Sadly I just looked online to find the correct MC to recommend to you and don’t find it. I see the disc MC but no all drum. It must be discontinued.
I used to work for the company that made Raybestos until I got laid off in 2005. Nowadays I look back and am bummed about how many parts and rebuild kits for our vintage cars I saw in our catalog when I was there but now are no longer available. How are we going to keep our cars on the road if no service parts like brakes are available?