I'm looking to find a trigger shifter for my son's MT60. The current twist shifter is difficult for him to turn and I worry about how he takes his hand off the handlebar and twists it from underneath. I've tried to get him to do it from the top but....he's 7. I don't want to change the Shimano TX35 rear derailleur or the 6 speed free wheel (Sun Run 14-28) unless I have to. Will a 7 speed shifter work? Is there a 6 speed trigger shifter out there?