I would be very leery of just throwing on an existing string set – Roosebeck or any other – unless you are sure that it was built with that specific set in mind. You do not want to over power the soundboard with too much tension, nor to have crummy tone with one that is too low.
A good set of custom strings from a professional such as Vermont Strings or Markwood Strings (for e.g., both US firms) should not cost more than about $200, probably less. Why take a chance and possibly kill the instrument?
As Tacye suggests, start by measuring the existing strings for diameter and, let me add, for vibrating length. This will give a professional string maker what they need to know for a replacement set. Assuming that you know the maker’s name, they may even have the original specs on file.
Best wishes,
Biagio