Began to change drive sprocket from 14 to 18 toothed on Puch E50. Looked simple enough: Remove circlip, yank part, reverse order. Got outside circlip off, but the sprocket did not release. Do I need a pulley to remove it? Do I need to remove the rear circlip also? Can I do this and replace the clips without a special tool or do I need to get something like a snap ring pliers? Any quick hints?