the horn a DC, I can't remember where I got it, CamLam maybe, but is small and fits by drilling a new hole in the fixing bracket to get it tight to the fork tube.
I was just looking at the cable and routing too. I think it's best to use the thickest inner cable possible to minimise stretch and elasticity to. I was going to mock up with bits of cable and then get Venhill to make a quality cable to suit. I had them make a clutch cable and it's superb. Good tip about the adjuster too, I'll get the cables made up with one, I may get a few made up if people are interested.