Welcome to the site !!
When you say valves ground and seats ... what exactly do you mean ?? , did someone grind the new valves ?? (thats a no no) , did a reputable shop that does dirt bikes cut the seats ??? , there are lots of reasons this could happen , but the most likely suspect is the valve keepers were not installed correctly , or the shims were installed incorrectly and the keepers popped out and the valves hit the piston and snapped off .... or the valve seats were cut incorrectly and cut too deep and the springs did not have enough tension on them to hold the keepers in place and the first time the engine was revved high they popped off and the valves hit the piston , .....
In any case , if the valves hit the piston you can kiss the rod/crank goodbye , as the bearing is damaged from the impact and you will have a rod knock if you put it back together without either replacing the entire crank/rod , or having the crank redone and a new rod and journal on the crank installed
More detailed info is needed .......
