Is there a rom hack that makes Kefka harder, but nothing else?

You could do that on your own. If you get FF3USme which contains all the statue bosses and Kefka you can make them more difficult by increasing their HP or the power of their attacks or their level and their enemy script. There is a tutorial to do this on this site, I would suggest you check it out.

Thank you!

I made a hack that does the following:

• Ultima is now holy-elemental (I couldn't find another way to make Ultima not work for this fight)
  • First phase is weak to holy
    
  • Second phase is normal to holy
    
  • Third phase absorbs holy
    
  • Kefka absorbs double holy
    
• Health adjustments:
  • If the enemy has under 32768 HP, double the HP
    
  • If the enemy has or is over 32768 HP, give Protect and Shell
    
  • If enemy is Kefka, set HP to 65535, give Protect and Shell, give Haste
    
• Misc:
  • Forsaken now ignores protection and split penalty
    
  • All instances where Kefka could choose to do nothing is replaced with choosing to do a normal attack
    
  • Names of the enemies making up the Statue of the Gods can be seen