I think I'd have a go at using flat stock to sandwich the engine plates and up the downtube.
Couple of clever breaks for offsets and some custom blocks to surround the tube.
There's plenty of room up front for heavy wall tubing, welded and press fitted into 3/16" steel.
Pass a 16 mm stud thru the center and you'll be good to go. Grab the front two and you're half way there.
I'm still thinking about the rear. Start with cardboard and move on to masonite as templates.
Took me 9 months of figuring to do my first and only sidecar. The Greevorex.
The second one will be easier. They built Zundapps with sidecar lugs.
Don in Nipomo