Welcome!
You have a few options. You can try surfing
www.taobao.com, but keep your search words very simple, like TK200GY-4A or just TK200GY.
If you can manage some fabrication, you could perhaps graft a rear section from a Japanese bike onto your frame. I'm betting that XR200 stuff would be a reasonable fit.
Search eBay, but not for Tank. Use generic terms, like Chinese dirt bike plastic.
Good luck!