You do not really need to back up your original files before using BytaFont and, starting from BytaFont 1.3.1, all the involved files are automatically saved as backup in /var/mobile/Library/BytaFont/Original

However, if this just makes you feel better, go to /System/Library/Fonts/Cache and copy the following files:

iOS 5.0.1 / 5.1 / 5.1.1 / 6.x:

• LockClock.ttf, PhonepadTwo.ttf, PhoneKeyCaps.ttf, PhoneKeyCapsTwo.ttf
• _H_Helvetica.ttc or _UH_Helvetica.ttc (depending on your device)
• _H_HelveticaNeue.ttc or _UH_HelveticaNeue.ttc (depending on your device)
• _H_MarkerFeltThin.ttf or _UH_MarkerFeltThin.ttf (depending on your device)
• _H_HelveticaNeueExtras.ttc or _UH_HelveticaNeueExtras.ttc (depending on your device)
• Georgia.ttf, GeorgiaBold.ttf, GeorgiaBoldItalic.ttf and GeorgiaItalic.ttf
• GeezaPro.ttf and GeezaProBold.ttf (Arabic fonts)
• STHeiti-Light.ttc, STHeiti-Medium.ttc, HiraginoKakuGothicProNW3.otf and HiraginoKakuGothicProNW6.otf (Chinese/Korean fonts)
• HiraginoKakuGothicProNW3.otf and HiraginoKakuGothicProNW6.otf (Japanese fonts)
• ArialHB.ttf and ArialHBBold.ttf (Hebrew fonts)
• Thonburi.ttf and ThonburiBold.ttf (Thai fonts)

iOS 4.2.x / 4.3.x:

• LockClock.ttf
• PhonepadTwo.ttf
• _H_Helvetica.ttc or _UH_Helvetica.ttc (depending on your device)
• _H_HelveticaNeue.ttc or _UH_HelveticaNeue.ttc (depending on your device)
• _H_MarkerFeltThin.ttf or _UH_MarkerFeltThin.ttf (depending on your device)

iOS 4.0.x / 4.1.x:

• LockClock.ttf
• MarkerFeltThin.ttf
• PhoneKeyCaps.ttf
• PhonepadTwo.ttf
• Helvetica.ttc
• HelveticaNeue.ttc

Then paste them to a safe location on your iDevice or on your computer.