I’ve always thought of “period” (or “original” or “authentic” instruments as we used to say) as being either the instrument the music would have been played on originally, or modern copies.
Many string instruments are in fact 18th century, but have had longer fingerboards and steel stings fitted, so presumably they could be (and in many cases have been) restored.
The idea as I understand it was to try to perform the music as it would have sounded, and as such I regard that as an interesting historical exercise, but not a guarantee of a better performance; better performances, I think, are given only by better performers.
Amongst the problems in “reconstructing” the original sound is timbre, e.g. compare the London Symphony Orchestra on recordings in the late 20s and late 60s; if clarinet tone changed that much in one orchestra in 40 years, how much might it have changed across a continent in 200 years? Another problem is what would the composer want if he knew his music was being played 200 years later? Would he insist we used hand-horns and pre-Boehm woodwind keys, or would he welcome the changes? Many composers did indeed welcome developments in instrument construction and design, e.g. Schubert and the piano in the 1820s.
So I’m far from convinced that “period” performance is always, or automatically, better. It can be, but it will depend on who’s playing.
Just to clarify, I think music is best heard on those instruments, and played and conducted by those performers, which bring out the best in the music. I think very often there was more in the music than could be conveyed by the instruments of the day.
This is why I think the late Beethoven piano sonatas need a modern concert grand, and Berlioz’ orchestral music sounds lame played on the instruments of his day; because in both cases the music was ahead of its time, in particular Beethoven’s compositions for the piano driving more robust designs for the technology of the instrument.
In terms of our own technology and the recorded legacy, the early performances of a work are rarely its best. I often think the early recordings of Elgar’s chamber music and Vaughan Williams’ symphonies, for instance, lack insight later musicians could discern, from familiarity. I exclude the composers’ own performances as presumably they had that knowledge which it took others a decade or so to learn.