I think that given it was 30 years ago and there's little in the way of solid evidence or first hand witnesses you're not going to be able to compile a convincing case. Also if your father was an employee of the mine then anything he created or built using company time and materials would ligitimately belong to the company as he was being paid by them for his services.
If I were you I would gather as much evidence as possible and maybe approach the law faculty at your local university and request some pro-bono advice. I wish you luck but I would advise you don't throw all you valuable time or money at this, it sounds ike too much of a long shot.
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