having third party property damage cover only sucks in this case.
Nothing really just cars can do.
the other company will use guides like redbook, glasses guide etc to see what your car "could" be worth.
however expect a low payout....
edit - payout - pretty much have to take it. you can argue the point if you have proof on what you spent, or maybe even approach VIP car inspections and tell them the car, mods, year, km's etc and for a cost they can do a valuation. unsure if its too late though...and again, maybe not even worth all the effort