First Build, Gary Fisher Level Betty. Which motor?

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In an ideal world, i would get a lifan 125cc, but unfortunately, there is no way to mount it to my bike! So, im probably about to amaze you with my stupidity, but this is what i need.
I need a gas motor kit (preferably 4 stroke) that mounts over the rear wheel instead of inside the frame. Also, it would be helpful if the entire kit costed less then 400.
people, please educate me in what my realistic options are I cant find much info on this style of mount.
 
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