I had a similar problem with my 1978 crawdad boat. We dicussed it and agreed to value it at my purchase price (in 1978) of $100.00 minus 10% per year of depreciation to a base value of $25.00 (current assessed value).

So you may try that with them.

Current value is purchase price minus 10% depreciation per year to currrent year.

I never registered my crawdad untill 1991. Most people did not register small row boats untill around that time. Then I read that all boats with any motor, even an electric trolling motor needed to be registered. Thats when the value issue came up.