My favourite aspect of these bikes is the ridiculously low parts prices.
To expand your search criteria on eBay, you gotta think like a person who doesn't speak English well, and who might not understand bikes. Search for items like DIRT BIKE MUFFLER, for example. Sometimes I laugh out loud.
I expect that your valves are too tight, and setting them will likely solve the problem. It'll be interesting to hear your observations about how it runs with the stock pipe and airbox, vs any mods that you make. Please let us know.