At the advice from some posts in this forum, I replaced the drive chain of my Motobecane 50V with a 420 chain, as the 418 chain is difficult to find. The chain fits on the rear sprocket without issue, but it does not fit the front sprocket behind the pulley. Is there something I can do to fix this? Is there an aftermarket sprocket that is made for a 420 chain? I didn't find a post from someone who had this same issue. Any advice would be appreciated!