I'm sorry for your loss.
I conduct probate valuations on a regular basis but for art and antiques, never an aircraft, however the principle is the same. A valuation for probate should be the lowest achievable price if the item was offered on the open market, for example at auction. Certainly not anywhere near retail value. If it was a car, the valuation would be closer to a 'webuyanycar' type of price. I'm afraid I can't suggest a firm to contact, but best of luck.