It'd be sort of odd for Buddy to have the masters, but it's not unheard of. While it's a slightly different situation, Ian Anderson (leader of Jethro Tull) had the master tapes for "Aqualung" buried in his garage for over thirty years, instead of it being stored at the record company's vault.
But even if the "Rescuers" master tapes were lost, there are always options. For starters, as Eyore said, Disney must at least have the mixdown master edited for the LP release. It should still sound just fine if put out on CD, though I suspect it's woefully incomplete from a score perspective. But there are usually multiple versions of "the master tape," especially for films produced in the 70s and later. You'd have the session tapes (the raw, uncut, flubs-and-all, multi-track tapes of the recording), the 2-channel mixdown tape (obviously a generation removed from the multi-track session tapes, this is the score before being dropped into the movie), the LP master tape (generally taken from the 2-channel mixdown), and probably some other dubs if they're being careful and want backups.
As a 70s score, I'd be VERY surprised if there weren't multiple sources they could go through for a Rescuers release. This isn't like Fantasia or Alice, where all the original master tapes for the scores were lost because Disney simply didn't have a proper archiving system yet.