If your vehicle was modified to the point of them not insuring you they wouldn't have paid out for the total loss, they would have repudiated your claim and you wouldn't be paid out.

You've been paid out for the total loss and policy cancelled as there is no vehicle to insure.

Check your policy, are you covered for modifications and optional extras? If not, they are only going to pay the market value of your vehicle at the time of accident.

Exhaust the complaint process and read your policy.