Is it possible to change the rear cog on the orginial Sturmey Archer 3 speed hub. I have alot of hills in my area, and would like to be able to get up most of them. Were would you buy a replacement cog, if available. The bike rides excellent on the flats and slight inclines, but when the hills come, I'm off the bike. Also how do you update the steering and keep the orginial fork. My folder is a 1972 Raleigh Folder.