You actually have a pair of options (at least) without retrieving the original disk.
One is to simply burn it to a double layer disk if your burner is capable (as most later ones are) since there is plenty of space on one for your project with no compression required, and the second is to burn it using Toast (and I am sure there are other compression options, but I have Toast and don't need to investigate other substitutes

) which will offer the option when preparing to burn to compress it enough to fit on a standard single layer disk.