Maybe Risse Racing has something for you? They claim to have a Sweet Spot compatible rear shock.
I don’t know what your budget is, but the ‘modern-brand-new’ bonustomato.com ride might be a Castellano. http://www.castellanodesigns.com/ He’s been selling Fango for a while and supposedly has a single-pivot full suspension bike. http://www.castellanodesigns.com/Zorro.html IMHO, that Fango is an excellent compromise that is mostly hardtail rather than the widely available full-cushion 2-wheeled ’69 Cadillac eldorados sold by the major brand names.
Otherwise, it all depends on your budget. I bought an old ‘sweet spot’ and a more modern fork and switched my old equipment over. I am quite satisfied with that and I’m still plenty fast (muhahahaha!) on such ‘old’ equipment.
I got an email back from John Castellano, aka “Mr. Pivotless.”
“I just made some pivot shafts, and I will soon have the bushings for the US
made frames. I’m custom-fitting some very nice German-made bushings to fit
Schwinn’s spec.
Give me a call or email in a couple of weeks and I can take your order.”
Naturally, I replied back that I was publishing this on bonustomato.com