Your Skylark is a GM X-body. This is the same platform used for the Nova, so a sbc should be basically a bolt-in using stock Nova parts. The swap will be easiest if you can find a 75-79 Nova for a donor. Later Skylarks were also offered with the Chev 305 and 350 engines, but Novas are just a lot easier to find.
What engine/transmission did your Skylark originally come with?