Pictors Studion has an English Civil War rules adaptation of Games Workshop’s Warmaster rules.
The Warhammer English Civil War group not only has lots of information for fans of the game, it also hosts a number of Warhammer variants, including an American War of Independence version.
The Jackson Gamers have this set of free wargamesrules for the English Civil War. They use a card activation system, (not surprising, considering the acknowledged Brom influence), and d6s to resolve actions. These rules are 20 years old and in their 6th edition, so they are well-tested.
The Lead Gardens Blog offers a set of iniative cards for English Civil War games. They’re designed to work with the Victory Without Quarter rules, but no doubt could be used for many others.
Mitch’s Wargaming and Model Making Blog has a short illustrated article on making an English Civil War style house. I’ve used similar techniques to build a town of houses for my Gothic Horror games.