How do you punctuate something you paraphrased?

There is no generally-recognised punctuation to mean "this is a close but not exact quote". The standard way of indicating this is...

To paraphrase Churchill, never have so many paid so little for so much.

...which clearly won't help OP, since it would take more characters to indicate what he's doing than would be saved.

There is a convention that you can replace part of a quote by some semantically-equivalent text in [square brackets] - for various reasons, including space-saving. So in this case OP could write...

"If I couldn’t do it [quickly], I couldn’t do it at all".

There are two common methods for handling alterations to a quote, depending on the nature of the alteration.

For omissions, the most common practice is to insert an ellipsis . . . in place of the omitted words or phrase. For alterations, such as to preserve grammatical flow in the context of a paragraph, to work around an omission, or to add explanatory or clarifying text, placing the new or changed language [in square brackets] is the standard.

That said, in the context of a tweet, if you're talking about committing a grammatical sin on somebody's behalf to fit within a 140 character limit, as in your example, I'm inclined to simply say: relax. It's not that big a deal, and changing 'within' to 'in' is hardly misquoting someone to the degree that it requires any consultation of a style guide.