Yes, you can sue for the difference assuming that you can establish that they really were similar vehicles. It's not unusual that the insurance company is low-balling you. Insurance companies don't make money by giving it away (although they still have a legal obligation to make you whole).
The FIRST thing that you need to do is to contact the insurance company and let them know the most similar vehicles you have found in the price range. They'll hem and haw over the price, maybe even "finding" other vehicles that are priced way less. Let them know that you can prove the maintenance and the value of the vehicle, and that you're willing to take it to court if need be as well as get an attorney. Ultimately the insurance company is looking to pay out as little as possible. Most people with vehicles as old as yours would take the money and put it into a different vehicle, where there's not a one-for-one comparison, and that's how they can generally get away with this behavior. But indicating that you're willing to fight and go to court for it means that they will most likely eventually cave. The reason is that they still have a "duty to defend" the insured. If you sue, THEY (the insurance company) has to pay for lawyers to defend the case. And if you have the evidence to prove that there's not a comparable vehicle for the money that they're offering, then you would likely win. AND if you had a lawyer, THEY would have to pay your lawyer's fees.
So think of this as an "offer" that you can reject. You counter offer, they say that you're ridiculous (or something along those lines) to try to get you to accept a lower offer. Again, insurance companies are all about limiting the amounts that they pay out to maximize the amounts that they get to keep. If you make it clear that it will be more expensive for them if you were to go to court than if they were to just pay now, then they're going to eventually cave.
Hope that clears things up a bit. If you have any other questions, please let me know. If not, and you have not yet, please rate my answer AND press the "submit" button, if applicable. Please note that I don't get any credit for the time and effort that I spent on this answer unless and until you rate it positively (3 or more stars). Thank you, ***** ***** luck to you!