Trying to fix my bike

[vreyes78]

Hi guys, I do not know too much about engines and bikes, thats why I really like this site. I had my gz 250 for three years and it was my every day ride, but it just stop working the last week. So I tried to fix it with patience, free time and the work shop manual. well this is what I got.
IMG_20160420_191408[1].jpg, IMG_20160420_185249_596[1].jpg

I Just wanted to ask if you think that I can still use the piston and the cylinder head? I need to replace the valves, do u guys know where I can find them?
Thanks for reading, I would really appreciate any comments.