Welcome to the site !!
I guess it depends on what your goals are for the bike ..... if its just for the kids to putt around , then you could save some money buy buying used parts on EBay , etc. , (head, cam etc.) , if it was lack of oiling causing the issue , then a whole rebuild would be in order , needing crank/rod , piston ,bearings , gaskets and whatever needed to be done (parts or machining wise) for the head ,or even locating (EBay again) a complete running used engine , so the cost can vary , and the depth of the repair/rebuild can vary greatly ...... Again , the goal you have for the bike (along with your budget) will determine what you need to pursue , as there are many variables here ....
So ...... Determine your budget and goals , then decide how far into this you personally want to go (doing the work yourself , farming it out to a dealer or machine or bike shop) , and what you expect out of the bike (trail riding or putting around the yard , etc.) , that will get you started , but there are so many variables , that there is no "do this" answer , without more info
You did not mention the year and that will play a huge factor in if its worth the cost versus cutting your losses and finding something else ..... again . many factors at stake here ......