ive seen some on YouTube. search "bmx motorized bicycle". the majority of them are rear mounted and looks like will require some machine work. it is possible but don't expect a kit or anything. or, you can look at the rear rack mount or friction kits out there.
Sorry let me clarify. There are many kits for rear mounted (above tire) for 4 stroke engines but for 2 strokes it will most likely have to be home made or custom. The mounting studs on the 2 strokes are designed to be between the bottom and seat post tube.
TBH, I would just go with a tried and true 4 stroke rear mount. The 2 stroke rear mounts are almost always custom and to be able to withstand the vibrations of a 2 stroke and overcome the mounts would require a lot of work. Of course, a 2 stroke engine kit is less expensive than a 4 stroke kit but for the amount of time and money needed to make a proper mount, I think I would just pay extra and take the 4 stroke. That picture looks like the 4 stroke engine is simply on a sturdy rear rack. If so then it really wouldn't be too effort to get a 4 stroke on the back.