There will be occasions when having a presentation ready to go at a moment's notice can come in handy: a prior client is hurriedly putting together a sales booster event and needs your special brand of punch, but you have absolutely no time between now and then to prepare; you're attending an industry convention and another speaker cancels at the last minute; or, a previous speaker on the same program misunderstands the requirements (or is given the wrong ones) and proceeds to deliver your entire presentation. And, yes, these situations do actually happen.
Step one: deconstruct. Take your speech apart using the Lego® approach. The pile that results will include variations on your opening statement(s), main theme elements, supporting facts, figures and citations, real-life stories that support the theme, and a few different, solid closings. When you're done, you'll have all the building blocks needed to put it back together quickly in multiple formats.
Step two: customize. Now that you're speech is scattered in pieces all over the floor (isn't that the way you started your play with Legos® as a kid?), you can reconstruct it in several different ways. Your goal is to plan for various audiences and time ranges. A good selection of times would be 30 minutes, one hour and 2 hours, being sure to allow for breaks between sessions and, for the long one, at the midpoint of the presentation.
Depending on the expected audience demographics, you might choose a rhetorical statement or poignant question versus a quick story to open the speech. Your technical facts and figures may be more appropriate for one audience than another. Some stories and anecdotes may appeal only regionally while the general public may find them awkward or irrelevant. How deeply you delve into your subject will be determined by the time you have to speak. And your closing would need to fit your chosen opening.
Step three: rehearse. Since you'll be delivering your speech with little or no notice, regular rehearsal is a must. Make a schedule to do a complete run-through at least once a month, especially for the longer formats; for the shorter ones, the more frequently the better. I would also recommend having an outline of each presentation handy, and reviewing them weekly. Want to look like a true hero? Keep some basic materials for the presentation in the trunk of your car.
As an established public speaker, you're (hopefully) always in demand. And now, when someone frantically approaches you for help at the last minute, you can calmly, confidently reply,
"Why, yes, I happen to have a speech right here in my back pocket."