Two ways I can think of; print concentric circles on a piece of clear decal paper, use the decal on a clear thin plastic sheet, align your piece under the clear sheet, then mark center with a sharpened needle in a pin vise.
A variation of the method posted above, is draw four arcs, location is irrelevant, as long as origin is on the circle and the radius is longer than half the diameter, connect the lines at the intersections, and that's the center.