gilera 125 4 strok engine tries to turn over but dose not fully turn over ?

their was a clicking from relay so changed that then changed starter motor found out that the sprocket by starter motor had sheared its teeth so replaced that but engine still refuses to fully turn over or start battry is ok bike was running ok with no probs till teeth sheared now it feels like a tight engine this is my sons bike and have run out of answers any advice thanks