I don't know that off the top of my head
这个问题我现在一时想不起来。
I can't answer your question off the top of my head.
我现在一时回答不了您的问题。
I can't comment on that off the top of my head.
我现在一时无法对此发表评论。
Perhaps can I get back to you on that later?
或许我可以过一会儿再回答您的问题？
Perhaps can I talk about that later?
或许我可以晚一点再谈这个问题？
Perhaps can I deal with that later?
或许我可以迟一些再处理这个问题？