Assume you have absolutely nothing except current year statistics. If you don't have current year, you should not be doing the return.
Reconstruct beginning balances by asking about real estate and equipment, and dates of purchase, then reconstruct accumulated depreciation. Current year cash can be rolled back to a Jan 1 balance. From amounts on loan service, you can deduct the amount interest, then reconstruct beginning balances on loans.
Current year information is necessary anyway. Make sure you charge enough and tell them up front it will cost.
For what it's worth, I do schedule L, M-1, and M-2 for every Partnership and Corporation regardless of size. Anything less means I am not setting forth proper due diligence.