silicone tends to not bond well to plastic, this does vary depending on the silicone - plastic.
You can weld plastic which would be the best option diy route but its not easy if you have no experience, the only other idea I can think of is metal welding a frame right around the rim of the tank & silicone this into place. A plastic glue may work but how supportive it will be I have no clue.
if you can replace the top maybe the best option.
Me personally if there is no bowing I would leave it but I am not exactly right in the head with these sort of things xD