yat if its cylinder is 72 then now its bore is 50mm and stroke is 41.4mm ,,thus making it 81cc bike,,
and your bore is way too much greater then stroke,, in my opinion,,so i dont know who told u to bore it so large,, and what else to do,, jis na ap ko mashwara dia tha us ka kia kahna tha?
by the way have u tried setting the carb? it needs readjustment,, u should checkj the plug if its ok,, then ur carb settings r ok,, if not then u may need rejetting and also i may nee to fiddle with the needle position,,