Bike noob here, I have a pretty old Suntour Fleetwing with step-over steel frame which I bought used/refurbished/multiple parts replaced from a reputable shop last week. Its 5 speeds with a single stem shift which I am finding really bothersome. Is it possible to make it to a thumb shifter instead? Id really rather not have to look down and remove my hand from the handle while driving. How much could I expect to pay for such a conversion?