Here is a John Blue gas powered roof cleaning pump http://www.dultmeier.com/products/search/7807
It is only 700.00, and bolts right on a small 2.5 to 3hp Honda motor, or you can use the harbor freight predator motor.
It is nearly 300 psi, so it will shoot a long way.
As a rule, I like air powered roof cleaning pumps better, because they are more reliable.
For a Gas powered roof cleaning pump, these are more relaible then a UDOR Roof Cleaning Pump, and cost about 1/2 as much.