Wanting my custom ringtone back, but how without erasing phone?

Yesterday i was running jailbroken iOS 5.1.1 & amongst other things i had a custom ringtone on there. It'd been that long that i'd forgotten how i even put it on there.

I updated to iOS 7 & i lost the ringtone off my phone which i'm now wanting to put back on but can't remember how.

I just tried plugging my phone in, opening iTunes, clicking the "tones" tab under my phone & Syncing selected tones.

I then get the message saying this is synced with another account (on my PC) & i can erase the entire phone if i want & sync with this library. Thing is there's so much i would end up losing.

So how do i get this ringtone on my phone?