Yes, buy a Muff’n (Q1 through Q4) and combine it with the Stage 3 Booster (that’s what Q5 is) and there you go.
Just put them in series and place the Stage 3 Booster last, or near the output.
It would be much more versatile to have a separate volume and foot-switch for the boost but you could also just use the volume of the Stage 3, (bypassing the Volume on the Muff’n as well as a bypass foot-switch for the Stage 3) to control the entire circuit as it is done in the CornishP1.
That is all it is, via the schematic.
I have no idea of anyone offerring a kit but you can do it on your own using our Combo Build info located on the Guides Page.