I'd like to find original papers by Ryll-Nardzewski and Svenonius on $\aleph_0$-categorical theory.
- Ryll-Nardzewski. "On the categoricity in power $\aleph_0$", Bulletin of Academy of Polish science mathematic 7, 545-548, 1959.
- Svenonius. "$\aleph_0$-categoricity in first-order predicate calculus", Theoria (Lund) 25, 82-94, 1959a.
- (Possibly) Svenonius. "A theorem on permutations in models", Theoria (Lund) 25, 173-178, 1959b.
On the webpage of Bulletin Polish Acad. Sci. Math, there are only issues after 2004. How do I get issues of older years? Is there a free version of 2 (and 3)?
Theoria is fully online it seems; Svenonius' papers can be found here and here respectively. These are behind paywalls, however, and I don't think they are legally freely accessible online. A library is probably your best bet (and for the Ryll-Nardzewski as well).