Hey Don, Sorry I was late!
Well, surprisingly enough I'm sorry I was late to the party on this one! Having had to remove a few of these over the years, you did the right thing. The pic indicates that NON- Distilled water was mixed with the Anti-Freeze. The Phenolic material that this pipe is made from is a Strand Reinforced Plastic. Unfortunately, minerals like Calcium and IRON attach themselves to the Strands at a molecular level, and cause the Phenolic material to expand. This is why you are seeing "Rust" in an Aluminum Block with a "Plastic Tube" inserted into it. If this is an MKI Bike, you are going to want to have a good close look at the Water Pump as well. My guess is you will put this back together with a new gasket, it will overheat, and you will think it is the head gasket again, when in fact it will be a bad water pump. Of course, this is just my Opinion. These engines are known for being dependable, so "Something" took that head gasket out. Here again, "Just My Opinion. It would be worth a water pump gasket to check the Impeller of that pump. Just an FYI, the mixture ratio for the coolant is 60% Anti-Freeze to 40% Distilled Water.
The other issue I have seen that will take out a head gasket is if the CDI from another Venture has been installed in this bike. (A 1983 or a 1991 to 1993 CDI will not function properly on a 1984 to 1990 bike) Just thinking out loud here.
Let us know what you find out Drew, you have peaked my interest. Oh, and don't be afraid to call "Bull S***" on me if you replace the gasket and it turns out fine! I am here to learn, just like you are, my friend! By the way, WELCOME to the Forum Drew, It appears as if it didn't take you long to get to the "Meat and Potatoes" of it all. did it?
Earl