What is the best motorized bike kit i can buy for around 100-200 dollars (also a couple extra questions)?

i was wondering whats the best motorized bike kit i can get for around 100-200 dollars. i have never built anything and dont have anyone who knows much about motors. i am looking for relatively easy directions. i have herd that 4 stroke engines are the way to go. I have a mountain bike that i could put the kit onto but would this work. Please help. thanks! i am willing to alittle more if theres a better one.