You did not tell us did you intend to do with it?
Glue the two pieces together. I would do it with UV-curing glue (need sunny day); do not use instant glue. Before you expose to sun, make sure they are precisely aligned, using the match of the crack surface and checking the surfaces and edges match.
- Either use as-is; at least do a first test
- Or, if you think you see objectionable flare, cover the region of the (glued) crack with a thin line of black paint. All you will suffer is a small loss of effecitve aperture (plus diffraction by the edges of the black region, but this should be negligible wrt te native geometrical aberrations).
When you finally re-mount the lens, use some foam or other between the ring and the glass; three small pieces in a triangle should be OK. Whatever method you use, make sure you preserve the centering as per the original design.