How to take opening balances when accounting for past years is not maintained...???

In the query please give clarity like any invoices of FA ia available & whether tax audit is applicable for the client & turnover of the client exceeds 1 crore or not......

If the client have the year of purchases of fixed assets then backward calculation can be done....otherwise you have take the help of valuator and get the assets valued...... Take the resale value as the fixed assets value..... 

Yes you can...........

But please once confirm with your client how much he wants to maintain as capital and take care about the cash & bank balance in the balance sheet of the client.