Screamin'Demons aka FS:SDOE has appeared as a bit torrent download, which I guess can be found with a search for those sorts of things. The d/l is a DVD image which installs the fully user-community-modded version, amounting to 2.7Gig, featuring many added terrains and over 250 planes including P51 versions B, C & D. Being originally released in early '99, it is very compatible with older machines.
Regarding scrambles, in quick missions the closest thing in BoB/BoB2 I guess is the Low Level Raid QuickShot/InstantAction, where flying for RAF starts with a scramble. Otherwise, choosing to fly from takeoff in the campaign will result in plenty of scrambles.