Side valve engines are very simple. A lot less moving parts. No timing chain, rocker arms, push rods..etc.
For trials applications, it is very attractive. The head is very compact and will fit better in a narrow trials frame.
The big disadvantage is, very poor volumetric efficiency. The combustion chamber has to be huge. The air/fuel mixture does not have a direct path into the combustion chamber.
Here's a side valve engine in action.
Scroll down to
The sv engine
http://library.thinkquest.org/C00601...motor.php3?v=2