If the records are incomplete, it takes some tolerance and patience to reconstruct a plan’s financial statements. I used the most recent information, which was fuller, and worked backwards—for some elements extrapolating an estimate based on the known amounts. For example, if your knowns are opening and closing balances, contributions, and an absence of distributions, one can interpolate an investment gain or loss. Or if an investment gain is known or sensibly estimated from mutual funds’ published information, one can extrapolate an opening or closing balance. My client had records on contributions.
Whatever your client estimates is better than not reporting.