Yanmar 2GM20F mixing elbow replacement

[dirkfr]

This is my first post, so please bear with me.
I am facing the necessity of having to replace the mixing elbow on the 1985 Yanmar 2GM20F that propels my '85 Pearson 303 when there's no wind. A couple of weep holes in the elbow and reduced raw water flow through the exhaust tell me that it's way past its life span. The double-threaded connector linking the mixing elbow to the exhaust manifold is the problem. So far it won't budge, despite generous applications of Goop and Gunk where the thread meets the manifold. Has anybody out there faced this problem and emerged victorious without seeking professional help?
Thanks in advance.
Dirk Ruemenapp, whose Pearson 303 "Loreley" is moored in Kittery, Maine.