Black tank toilet plastic piece

On my new-to-me '71 AS Sovereign, I noticed the original Starlight toilet rocked a little bit. I removed the toilet and unscrewed the flange. It looks like the white plastic piece that is glued to the black tank has lost its grip on about half the circle. The tank has two very small cracks at about the 2:00 and 7:00 o'clock positions, about ½ inch long.



I'm planning on changing out the toilet to a Bravura because at some time someone threw a cigarette in the toilet and it burned the bowl. If it wasn't for that I'd probably change the seals and keep it.



Is there some kind of glue I can use to stabilize the cracks and bond the plastic circle back on the tank?



Thanks!