It is not uncommon that the muffler assembly is the first thing to go. Ours lasted a year. An entire new muffler/head pipe assembly is about $60 or so (+/-). Check with Tom at SYC Power sports. He can get American Sportworks parts at the best prices I have seen.
For the throttle cable, it is pretty easy to replace and it too is pretty cheap. Another common problem. Ours hasn't gone just yet.
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