Vauxhall Astra engine starts but car doesnt go

hi there hope someone can help i recently drove over something at low speed which pulled off my exhaust and punctured my petrol tank....my mechanic has ordered a new fuel tank from vauxhall which is apparently a modified version of my old fuel tank...basically he has put in 2 gallons of petrol started the engine it starts but then it seems like revs arent goin up as if there is no petrol in it?? something like that im just tryin to relay what he said...could it be that the modified version isnt compatible with my car...apparently there wasnt enough pipes with the modified tank and he had to use pipes of my car to help with the modification