The engines were originally designed years back for what I call the "double diamond" frame.(see pic)
As has been said here there are so may ways to mount to different frames, and many mounts available it basically your pick. Just choose something with enough room for the engines dimensions. A lot of MTB frames are pretty tight. Generally most cruiser frames will require and extended front mount, which are easily found. Depends on how much work you want to put into getting it mounted.