The simplest frame is a flat plate (skid) with holes drilled through for the pump and engine to be mounted on.  These work fine if the unit is to be mounted in a fixed position.  

If you need mobility then the typical cart used for up to 13 HP is a two-wheeled type frame which can be tilted back and wheeled around.  The two wheel cart is very maneuverable for getting around corners and into tight spaces.  For 16+ HP machines a four wheel cart is needed to carry the weight.  Pneumatic tires make the pushing and pulling very easy but you need to keep the tires pumped up.