Everything needed to calculate the business's profit. Records of how much the business's receipts were. Receipts for all business expenses you want to deduct
I think the best thing you can do is consult a professional accountant. I think your tax liabilities are more based on when you actually incorporate and not so much on the Employment Identification number. I would also study and research everything about the topic I could on the IRS website:
It never hurts to really educate yourself from the source about the subjects you are curious about.