Metasite is a material with unique electromagnetic properties. The electrical forces holding its atoms together are very malleable when subjected to different
There's more, the conductivity or metasite can also be tailored with the same magnetic fields. It can be a nearly perfect insulator or a superconductor holding vast amounts of energy. The energy can be released again by spinning the metasite through a specific magnetic field and the energy will be released in whatever form desired: em, nuclear, repulsive or gravitic.
Space craft can use the repulsive or gravitic properties to travel interplanetary distances with travel times of weeks or a few months, thrusting against or attracting the sun itself and (mostly) free of the need for propellant. Most vessels do have more conventional rockets for high accelerations.
The amount of metasite and the efficiency of the engine determine how much energy can be stored. It does provide unlimited delta vee because of these limits. It also has a nasty tendency to build up static charges and ships may need to stop somewhere to discharge this build up.
I came up with metasite because I wanted a dieselpunk style technology and that means it uses the technology of the 20's and 20's as its template. One f the first things you'll note about that era was the technology was not capable of sending a ship any farther than the Moon and that might be a stretch. Therefore we have unobtainium allowing 30's technology to exceed 21st century technology in several ways. Those ways are material science, energy storage and propulsion. That covers most of your space faring dieselpunk antics.
It is improbably useful stuff. I don't even want to hazard a guess how it is produced or mined (or whatever). You make up something that fits your world. Is it common enough to make flying belts affordable? Again up to you. On the bright side, one type of unobtainium with these related effects seem more coherent to me than separate advances providing for antigravity, death rays and space flight but not smart phones, price scanners, and the internet.