I have this sentence:
This is all I can say.
I have to turn it into a negative statement.
I have tried a few things and ended up with:
I can't say more than this.
Am I right in rewording it like this? Are there other possibilities?
This is all I can say
"I cannot tell you more either because I do not know or I do not wish to (do so)."
I can't say more than this
The meaning is the same, and it is now a "negative" [grammar] statement. So, yes, you are correct.
Other ways to do so? While maintaining a "negative" structure:
I can say no more than/of this.
I cannot elaborate any further.
There are no other things to say (on the matter).