Margus, if you’re in the neighborhood of our shop you could stop by and take a look at it. From what you describe I think we have what it would take to sort it out or at least give you an accurate diagnosis of the problem.
Granted we can’t fix it or lack the parts, we can direct you to very nearby where a strip of bicycle shops carry most parts you’d need to get up and running.
In case you don’t know we’re at 小天东街3号附26号 (3 Xiaotian East St., Unit 26) or if you’re trying to find us, from the intersection of 玉林北路 (Yulin North Rd.) and 一环南三段 (First Ring Rd. South 3 Section) go north and take your first left onto 小天南街 (Xiatian South St.) and walk about 100 meters. We’re on the south side of the road!
Good luck!