We have one member,
@yuckfoo, who went through hell and high water trying to adapt a motor kit to a Mongoose multi-speed bike a while back...Many, Many, months of failure and many dollars later until he finally succeeded, and he is very mechanically inclined...He has not been on the forums for a while now though, so hope all is still going well with him and his Mongoose...lol.
This was his finished product video once he finally got it together and worked out the bugs on it....It would not surprise me if he has a couple of thousand into this after all the hassle and heartache it also cost him to build this.