I'm replaced the Simplex front derailleur on my UO-8 with a Suntour Compe-V and it's been working flawlessly. Among many others that should work, the Suntour Spirt is another cheap and plentiful option.

If you want to try keeping things original, I still have that front derailleur from my UO-8. It's intact and I'd be happy to give it to you.