By the way, I just converted a Lobaugh Berkshire to 3-rail for a guy. We used a Williams mechanism, and just cut the tailbeam to accommodate the four wheel Berk truck. Bolted right in. I wonder if you could do that - just buy the Williams "J", bolt the mechanism into the Sunset boiler, and sell the leftover pieces as a beautiful 2-rail locomotive ready for assembly. I bet you would break even, except for all the expensive electronics. I might even bid for the leftovers, and I am all through adding to my collection.
I have thought about that option. I saw a William J on eBay not too long ago for a decent price and almost bought it. I may look for another before buying the Lionel parts.