Title says it all. There’s barely anything on the web that talks about Reagan post presidency, let alone his reaction to 9/11. What did he say or think, if he thought or said anything about it.
Bonus Question: His reaction to USSR collapse? Same little amount of information on his reaction to the USSR’s collapse.
Ron Reagan's My Father at 100 suggests his illness was too far along to comment about 9/11. (EDIT - to clarify, 9/11 is not mentioned in the book, but Ron's description of his father's decline gives a firm answer to the question between the lines, imho.)
The former president took a fall from a horse in 1989 and needed surgery to sort out a blood clot between his skull and brain. In 1990, he started annual mental acuity tests that showed evidence of Alzheimer's in 1993. His last speech on 2/4/1994 shows some noticeable decline at the beginning, then he slipped into gear and performed quite well. His public announcement came later the same year. He died 6/5/2004 with the immediate cause being kidney failure, but he would have been laying in a bed and not functioning much mentally for some time by then. Long story short, he is unlikely to have commented on the subject. Still, it is possible that there may have been moments of clarity on a good day. If others have something more optimistic from another source, I'd prefer to be wrong.
Bonus - to address a related question, the son Ron hems and haws a bit, but he decides in his book that President Reagan had no Alzheimer's symptoms during his presidency. Ron noticed a significant mental not-quite-rightness especially in how his father handled Iran Contra around 1986, but in Ron's opinion, that was a 76 year old man being 76, says the son Ron.