I want to use 5e as a base for a small science fiction campaign with a few friends...

I guess you could just flavor text sorcerer into a psyker of some sort, then just kill wizard, warlock, and just keep it low level I guess? Kinda ruins the point of D&D though. Can't you just make your group less lazy little shits and have them read a damn book?

The only hard sci-fi you're going to be running in 5e is the softest fantasy in space shit this side of Star Wars, dude. 5e doesn't have rules for spaceships, sensors/communicators, reaction drives, newtonian space flight/delta-v, atmospheric pressure or atmosphere in general, radiation, space adaptation syndrome, high acceleration, the bends, anything. Do you know how many rules you're going to have to make to run this?

Basic points of the setting
-ringworld megastructure
-PCs are either born there or moved there
-no known alien races
-cybernetics but not necessarily chainsaw arms
-space poverty
-AI/supercomputers
I was going to focus on mysteries on the ringworld, conspiracy, and poverty.

Basically, what happens if everyone gets to space, but there aren't enough jobs? What happens when people start stealing on a giant mega structure with a relatively small government and therefor small police force?

assuming no magic the only classes you have to work with are rogue, fighter and barbarian, maybe port dandwiki.com/wiki/UA:Generic_Classes#Generic_Classes
to 5e?
Also you would need to swap out the weapons for the setting equivalents

Plays Stars Without Numbers instead. Capable of hard sci fi and has classes. It has quite a few useful supplementary books too for playing characters from all sorts of backgrounds.
If you want a great example of a SWN campaign, check out Rollplay Swansong on twitch/youtube.

Pirate some old D20 Modern books, flip through them and take some hints. They broke off with classic D&D class system and replaced it with 7 Ability-based classes though.

*ding ding ding ding*
Tell him what he's won!

Quit repeating your lie at every opportunity.

It's not a lie though.

you have to ask yourself if this is the only time in your life that you want to run sci-fi/cyberpunk. if the answer is no, learning a new system is a good idea.

If you're going to do hard science fiction I highly recommend you try out either Diaspora ( vsca.ca/Diaspora/ ) or Orbital 2100 ( drivethrurpg.com/product/193706/Orbital-2100?manufacturers_id=3743 )

I don't know, access to a system that is better suited for their request then what they actually though existed?

It is though, and you know it, but are too afraid of D&D overlapping your favorite game and snuffing it out for you to stop repeating that lie.

*thought

there's no reason to frankenstein together a hard sci-fi RPG out of a high-fantasy system when there are already sci-fi RPGs out there that are specialized to exactly the setting you want to run, and will work twice as well with half the effort. Yes you could do it, but it's a colossal waste of time.

I guess it makes sense if you don't have the money to buy another rpg book and you're too moral to just pirate one

Check out the Unearthed Arcana article for the Mystic class playtest, and use that as your primary spellcaster class. It encompasses the psionic classes from previous editions and generally narrows them down to a more psychic feel; more mind powers, less generic spells on a point system.

Maybe replace spellcaster classes with "magic" items. That way you could limit the spell list to things that you can refluff and explain in a scifi setting, while also signifying how accessible technology has made such wondrous artifacts.

Maybe go the Numenura route and explain away spell effects as trained technicians manipulated complicated nanomachine swarms.

Can't tell if lazy troll or simply delusional.

...

lazy troll

>I use D&D to do something it was never intended to

Just kill yourself instead. It'll be less painful(at least for your players).