Without doing some tests, it's impossible to say whether the conduit will provide a functional earth. That's the only way to be sure.
You're OK with plastic fittings, but metal ones present the possibility of the light becoming live in the event of a fault. If it's not earthed, then the breakers/fuses would not operate, and the unit would be live to the touch.
Really, the only safe option if the conduit can't be used, is to take a new earth cable back to somewhere with a known good earth connection.
I know the likelihood of a fault is remote, but I'm afraid it's the only safe way.
You could use a metal fitting if it has low voltage lamps, because a transformer would not have, or need, an earth.