Retrotec got its start in single speeding. Bob was a huge fan and promoted it whenever he could. Bob put on the first WHIRLED race in 1995. I have tried to continue that legacy by helping SSWC whenever I can.
We can build any style of Retrotec you want into a single speed. We prefer to use a Phil Wood eccentric bottom bracket or PF30 with a Beer insert. They work great and keep the lines of the bike uncluttered.
On Inglis frames we can use EBB, PF30 or hooded sliders from Paragon. As most people seem incapable of riding a hard tail on dirt anymore, why not really blow their minds with how useful a hardtail single speed can be?