LF353 -- 4560... both are dual op amps and are more or less interchangable. In fact I just installed a 4560 in place of a LF353 last night in a GK amp. Works fine.
Look at your old schematic. What in there uses that -5v? What now does that same job in your newer product? Without looking, I'd be betting the -5 was part of some FX circuit, maybe a chorusing circuit.
Otherwise is there great similarity? Enough to steer you through?
And do give them a chance to correct their oversight. You told them your board number, right?