Yamaha dt125r with problems

I brought a j reg yamha dt125r For 550. It was in good condtion cosmeticly with a dep exahust sytem. 30miles after riding it, it just cut out Took the head off and was told to replace the piston but it wasn't seized. I was also told to get it running and sell or rebuild the engine because soon it would need a new big end baring. I left the bike dry stored for a week and now the conrod wont move up and down and part of the crank ( the big cricle things beside the conrod have flat spots on them and feel slightly bumpy. My question is what is wrong with my bike. Should I fix it or sell it as a non-runner (I can get the engine spilt ext without taking it to a garridge). How much Would it sell as a non runner it has long Mot and tax. And where can I buy a full set of seals and all the little bits to rebuild the engine.