To the question about cold starting, as in Winter weather, I've hand cranked the B&S flatheads (meaning the little 5 hp engines that are common-place to everything) with the temperatures in the 30's DegF. No gasoline or any other starting fluid. I've never tried alcohol on a Flathead V-Twin. And, I don't remember the compression ratio of a typical B&S 5 hp. But, they ran strong, and they ran clean.
As far as changing the effective compression on the Flathead H-D's, degreeing the intake cams to close a bit sooner (also, meaning that they open a bit sooner) is easy enough, though I'm thinking a special grind to open them at their OEM spec, and close sooner may be better. Maybe someone has tried this. I'm thinking the stock ones will run ok, for a trial.