have you checked the front sprocket tho? That rear sprocket can definitely go on one more set of chains. If you do decide to change it, I'd suggest upgrading to a lighter sprocket. I don't know what kind of noise you're hearing but mine was having clunky noises until I brought it to the shop and find out it's a little tight. Small symptoms of these bike are really noticeable, if something's not working right the bike will tell you.
Riding the childhood dream