Technology in Skyrates is a mix of things from the early 20th century and some "weird science".
The following was taken from the Roleplaying Guide and is generally accepted as Canon:
Inspiration for Skyrates was drawn heavily from the TaleSpin TV show and the movie Porco Rosso, so the setting drawns heavily from these stories.
The world exists paralleling the atmosphere of the 1940s. Although guided missiles and jets are specifically disallowed, accepted canon technology is limited to the year 1959. This is a limit, not an average. Most technology should feel like the 1940s.
It is hinted  that other technologies may have been available before the Great Upheaval, though details are vague at best ("War machines dragged themselves across the earth[....]") Any discovered through Roleplay would likely be ruined and difficult to understand.
The Hidden Fleet possesses technology that is not available to Skytopians, most noticeably stealth technology for their aircraft.
Unobtainium weapons are canon, though they are not in use today due to their destructive power. It is not clear if the knowledge of weaponizing unobtainium even survives. Unobtainium is used in more peaceful applications, however.