Seaseng 125 6ssgx engine problem

My boys were riding last week and it got hot and they said it started smoking out the botttom and exaust just before it died. The engine tag says its a model 6 ssgx 125 ach.
I am guessing the motor is blown. It wont even try to start. Its getting Gas, Fire and good compression. Checked the vavle settings and they are right at .004. The timeing has me wondering due to the fact the timing mark can be dead center but the mark on the  gear that drives the valves is at about 9:00 with the top valve closed.
Can I get a rebuild kit for this engine or where can I find another engine to replace it. Also looking for a engine manual for this model.
Any help would be greatly appreciated.  

the egnines can be bought for around 200.00 new just do a search on seaseng, or better yet loncin engine

the loncin engines are made in japan not china. so they tend to be alot better.  
Keep it RUBBER SIDE DOWN!!

125cc chinese atv engines are more like $300+ plus shipping, so expect to pay at least $350 for a stock engine. if you have a rear foot brake pedal instead of a hand lever,  for the same amount of money i would recommend buying a big bore manual transmission engine off of e-bay,then adding a clutch cable and lever to the atv. you will have a much faster and better running quad.  
07 prowler

oh my bad, i was thinking 110cc, yeah the 125's are alittle higher.  
Keep it RUBBER SIDE DOWN!!

Thanks Wolf and Bill, I am going to start checking around for another motor. I did pull the valve cover off of this one and sprayed a little starting fluid into carb. It makes a sound like a pop and it has a white/bluish smoke coming out of the valve cover hole when you turn it over. Still doesnt even try to start. Do you have any suggestions ? I have 3 other chinese atv's and would like to learn a little about these engines. ( The others are 110cc ) I figure even if I have to replace this one, I can use it for learning and rebuild it with my 12 year old son.