Level it and create terror.
Absolutely the potential would be there to recreate power, and possibly to eventually refine fuels, although as stated fuels might take years. Crude chemical weapons would not take years.
With electricity alone however, you have thermal imaging and light amplification. Never let Roman forces rest at night. Even fabricated crossbows are going to wither forces who can't see.
When does the film hit?
All it would take is one guy capable of making BIO-DIESEL in a garage.
Military engines are designed to run on all types of diesel, not just the pure kind we have in the states.
I don’t think the night vision advantage can be emphasized enough. These things don’t take much power to run and are pretty rugged, and ancient people were very superstitious. Having to defend everything 24/7 would be very debilitating and demoralizing. You can’t fight what you can’t see.
It would be very disconcerting to find every single sentry around an encampment dead or missing night after night after night, with no apparent struggle.
Control Rome?
Can you imagine the abject terror of being surrounded by tanks, buildings exploding around you and seeing unknown attack helicopters for the first time sending down its own dragon-like flame on the city?
They’d be begging for terms of surrender.
even their spotlights at night would be frightening to the Romans.
If the vehicles run on diesel, you could probably rig them to run on biodiesel. Failing that, you can make ethanol or methanol. I have no idea how long a modern military vehicle could run on that kind of stuff though. They’d probably be better off with old Russian vehicles if they had to try that.