We had a spring teaser here yesterday with temps in the 50's so I spent a few hours in the garage. My frame is back from having the down tube welded in and it looks great. I will need the extra space afforded by the down tube since the exhaust points straight forward on the Villiers. That little engine is pretty heavy so having it low in the frame and close to the bottom bracket should help make the bike handle decent. There's not enough space to squeeze the Albion in between the engine and the seat tube so it will need to go between the seat tube and the back wheel. Space might be an issue so either the frame gets extended or I look for 24" wheels. Maybe both.
The next one step forward, two steps back thing is the fork and headset. The Sears frame has the head tube lugs extending inside the head tube. I'm guessing this was a cost thing to keep it cheap when it as made but the flanges in the head tube interfere with the 1-1/8" fork. Looks like I'll be shopping for a new fork and headset. Good thing this nice weather isn't here's to stay or I'd be getting anxious about progress. While it's still cold I don't mind spending my tinkering time on the Internet.