The safe answer is indeed a new line set.
On the other hand, it that worth it for a little-produced, little-known canopy from way back, from a company that doesn't exist any more? (Correct? Flight Concepts became Parachutes Direct which is also gone now?)
Otherwise if one wants to test pilot a bit, one could start fiddling with brake settings to affect the opening. Do the brakes need to be set deeper to try to catch air more, like pulling on rears? Although if the lines out of spec are "all over", then it isn't a very consistent situation.
(How far is "out of spec" for that company? Is it something that was in the manuals?)
I don't like providing simplistic answers, but I kind of think providing your customer with links to facebook and dropzone gear classifieds pages may be appropriate, to find a new used main parachute at an affordable price....