Proviso on Levels: The range and starting point of levels must fit the setting. You can start higher for more superheroic-level play. You can advance farther, if the basic theme is a grand cosmic battle.
- Square-jawed '50s star pilots and space cadets? You want levels 1 to 5, but probably no higher.
- Beginning Jedi? Keep the start point at 1 or 2, but the range has to be higher, perhaps 15 to 20.
- Lensmen? Start at 10 to 12, but top range, if any, is in the 20s to 30s.
Skills: I've already ranted about skills, so I'll just say here that you could get away with using the AD&D 1e secondary skills re-skinned for the setting. No skill rolls, no huge bonuses, just a "oh, you know how to use mining equipment, because you were an asteroid miner before you joined the star patrol." At best, skills should only add a +1 when a roll is involved; most of the time, however, they only determine whether the character can do something or not. I'd use my backgrounds system from Blanc to handle skills.
Reward System: As I've already said, I'd use the existing XP system with tweaks in a sci-fi game. I feel those tweaks shift the focus from dungeon crawl to planetary defense and rescue. I'd also use experience based on ability scores: risky tasks are 10 x relevant ability score(s), ongoing situations where no rolls are made (like travel) get 1 xp per ability point.
So, as it stands, I technically have enough stuff for the basics of a space RPG: Liber Zero, plus Blanc, plus modified classes and my xp house rules. All that's really needed is some setting-specific lists and rules hacks for spaceships and other sci-fi gimmicks, including any rules for creating appropriate adventures and situations.