Yes, there are several ways to restrict the flow. The easiest way is if can find another nipple fitting and put some solder in it, then drill it out to the size that you'd like. Do this on the intake side only. One other trick is to use a wheel collar and use the screw to adjust the flow. You do have to watch that it doesn't get a hole in the line.
I've got 3 ponds nearby that I run at. You'll have to let me know when you're in the area.