Carl,
Are there any stump grinder dealers in your area? If so, you might be able to purchase the hydraulic head unit from one of the units that will match your GPM and pressure, or find a used one. If you have to make it your self, you will have to do a bit of machining, and then you will have to set up the teeth at the correct angle, and spacing. Although a machine shop can do all this, they aren't cheap. There are a lot of different teeth out there.
You probably know that I have the PT stump grinder/trencher, but I am planning to mount it differently so it will cut/grind left to right. not back at me. I also have a tow behind small stump grinder that uses a Honda 13 HP engine to double belt drive the cutting fead which is about 12 in in diameter, with eight carbide teeth. Although it has fewer teeth, the head turns about 2000 RPM, and it does a credible job. The teeth are only a couple of dollars apiece.