Lyre Harp (Birch Hill RG12)

Musical Instrument project. Needs restringing. The original strings are old and inclined to break when trying to tune it. Tuning key is included. Three strings have broken in total. I have tried using a guitar string as a replacement on two of them but it is too thick but maybe there would be something thinner that would do the job. Nylon string could be used but it would sound much better with metal. Only one thickness is required because the pitch is controlled by the diagonal wooden bridge. I have included images of the areas where the strings need attaching.