A wavefolder? Definitely a big part of the Buchla sound, and it'll be perfect for those 258t sine waves. Intellijel make a pretty fun one, the Bifold, but there are multiple options.
A 296-style resonant filter bank/EQ? That's a pretty classic west coast design. The Buchla one may not fit, but the Serge or Erica Synths ones will. ADDAC also makes a triple bandpass filter like the one Buchla makes for their 4U systems, which might be a more useful version of the concept for you.
Some of the recent module concepts from Tiptop like the programmable pulser or the frequency shifter might be fun here.