I found a couple of 3.5 GPM at 4000 psi made by EASY KLEEN with a 13.0 HP Honda but I am not sure about the quality of EASY KLEEN products. General triplex plunger pump with stainless steel valves and brass manifolds. They are priced at $2200 each.
If you are going to clean concrete you need at least 4 gpm/// I perfer whitco (all belt drive) and 110v on board generator.. that powers the burner and has an extra recpt for lights and such... all for about $5800