My son has a 6-speed 20" Muddyfox Octane Rush bike with a sunrun rear derailleur. The twist shifter is broken on the right handle. I want to replace the shifter and need to know if it has to be a certain type. Will the shift ratio be the same if I buy one off ebay for a 6-speed? Any help/advice is appreciated. I'm a n00b when it comes to bikes..