Found the same Marinco kit for a lower price and free shipping -- at Amazon.com
Also found the spec sheets at Marinco.com, so I can check the dimensions to see if it will be easy to replace my existing port.
Well, I decided I like this so much better than what I have, that I'll just get it and worry about cleaning up my old installation later. Worst case, I may have one small hole to plug with a rivet or Bondo or something.
So I went back to amazon.com, and happened to scroll down on the product page, and found one by Minn Kota that's not only less expensive, but appears to be identical, just rebranded. It even shows "Marinco" embossed on the plug in the product photo (looks like even the Minn Kota Motors site has this same photo). So I ordered the plug/receptacle kit, for $30 shipped. Link to the product page is here