I've found another load of rules on my system, this time for a simple science fiction D&D game I had in mind. These were my system notes but I never used it - I don't even know if these rules work or not, but the starship creation rules had some promise and I might see about adapting this for D&D 5th Edition.
A VERY SIMPLE SCIENCE FICTION RPG USING THE BASIC D&D SYSTEM
Roll 1D8 for hit points, or 1D4 + 4 if you want better odds.
The saving throws 'Fortitude', 'Reflex' and 'Will' are scored at 10 plus the applicable attribute bonus. Fortitude uses the Constitution bonus, Reflex the Dexterity bonus, Will the Wisdom bonus. A successful saving throw is a D20, scoring less than the saving throw score.
Weapons remain the same, use bow and crossbow damage for the pistols and such.
Also, each point can be spent on a single skill, any type of skill they think they need, and the DM/player decides what attribute the skill falls under. Once they've decided, they get the skill at that attribute's score, and they have to roll equal to or under that score on a D20 to succeed in a skill roll; the score can be modified up or down depending on difficulty.
Some example skills and their equivalent attributes (suggested - you can change these if you wish):
Bargain – Charisma - The ability to barter, haggle and trade with other survivors. A successful roll means you get what you need for less than the asking price. This skill cannot be used on fellow PCs.
Climb – Strength - The ability to climb trees, cliffs and rubble. Difficulty depends on handholds and angle. A 45 degree angle with plenty of handholds is much easier than a 90 degree angle with very few places to grip.