Was Congressman Larry McDonald Assassinated?

In a column on why it makes sense to vote for the most incompetent, Gary North also makes a side comment about Congressman Larry McDonald:

[Congressman Larry McDonald] was the most conservative Democrat in the House in 1976. Arguably, he was the most conservative House Democrat in the twentieth century. He was a member of the John Birch Society, and had he not disappeared, along with the never-located Korean Airlines Flight 007, in 1983, he would have become the head of the JBS.