They all are "make it fit" installations.
In referance to the popular in frame 2 strokes:
Some folks have issues with the frame tube diameters (too big around) to fit the motor mounts, and there are adapters and other persuasive means to get them to fit.
Some, not all, of the newer engines now have slightly larger mounts to fit these frames. It seems like the Chinese have gotten wind of this common problem and have changed the mounts accordingly.
I am sure the other folks on this forum can suggest an engine for whatever frame tube diameter you are working with.
Browse through the 2 stroke, 4 stroke, and click here:
Mounting Techniques - Bicycle Motor at Motorized Bicycle Forum sections here and you will get the general idea.