coachbyrd,
I'm not really sure what you want to do, so perhaps this will be pretty wide of the mark....
By themselves, the two foulpoles don't have enough information to allow you to find the original location of homeplate. Actually, any point on the circumference of a particular circle could be the point of homeplate, and lines connecting that point to the two poles would make a right angle with each other. That particular circle has a diameter which is the line between the two poles.
Maybe that's fine and you don't mind if the new plate location is a few steps toward either dugout from the old position. If so, just find any point which yields foul lines which are at a right angle, and then layout the infield.
If you really want to find the location of the old home plate, you'll need additional information. Coach2709 implied one way--recall where the plate was in relation to the backstop. Lots of backstops are symmetrical, and the plate should go in the middle of the arc.
If the backstop is gone, you can probably use Google Earth to get an aerial picture of your field as it looked a couple of years ago. The program has a measuring tool that would allow you to measure pretty accurately the original distances from the foul poles to home.
Finally, if you can afford $30 or so, I'd recommend renting a "builders level", with tripod. Builders levels will allow you to measure 90(or any other) degree angles very accurately--much more so than string, especially if the wind is blowing. You'll also be able to sight the line through the middle of the rubber and second base. The details of where the bases go, as mentioned by MidloDad, are illustrated in MLB rules.