When I was young (a very long time ago now) I would have loved a project like this! A chance to have a go at instrument building, with a very good chance of success, and guidance available. I can't find any reference to Tardini, but the kit seems to date from the 1960s or 70s.
I would actually call this a single-register harpsichord, but it makes no difference. To avoid the complication of a bentside, it has a pentagonal outline, similar to the famous Zuckermann 'Z-Box'.
The original owner got as far as making the basic structure, but sadly was unable to complete the project.
All of the necessary parts, including strings, wrestpins, and jacks appear to be there, although a builder today might prefer a more authentic type of wire. A paint finish would be appropriate, starting with a basic 'Farrow & Ball', with the option of adding more elaborate decoration later.
The spinet kit is in the village of Escrick, south of York; the price is £100. Please contact the owner, Cathie Dance.