A TLVH431 (or TL431) which is a transconductance device can simulate the feedback loop for a MC34063.
defaults to 9V, add 43.2k for 12V or 8.45k for 24V, and both for 26.6V.
The need to bias the TLVH431 bandgap reference requires a voltage drop which I show with a Vb-e multiplier, and a 80uA current source. The reference input will not work unless the bandgap has bias.
