Elves will get a Trireme, a Galleass, a Work Boat, and a Caravel... that could be either the Trireme or the Galleass, but I'd lean towards Galleass because of the size.
Perhaps you could convert that, but also do a single-masted version to be the Trireme? Or I could try it... though ships are my nemesis.
Trust me, I am having terrible issues with ship conversions myself (I am still trying to complete a stealth destroyer I obligated myself to from the unit request thread - and having a difficult time with it

Elven Bombardament Ship

Elven Storm Ship

Elven Transport

Plus the previously mentioned Elven Warship. Removing a mast should be easy if you want a "Trireme" version; I can make that happen.