Buy a used copy of a 3.5 players handbook, get a permanent marker, and make some changes: Everybody gets 15 hp. If you're feeling reasonable, fighters/barbarians can have an extra 5 hp. Nix iterative attacks for anybody except the fighter-style classes and monk, give those guys a max of two attacks per round.
Feats: The toughness feat can either impart an extra 5 HP or you can eliminate it (if everybody takes it, it's a feat tax). Maybe stick power attack with a static -2/+2 to hit/damage, rather than a flexible amount.
Zip through the spells, blacking out and re-writing the damage amounts so they scale from 1d8 to 3d8, with steadily increasing damage and flexibility, or else black out the damage spells after a certain level.
I'd have baselines like:
- 1st: 1d8 damage, save for half. Magic missile works the same but cannot have more than one missile per target.
- 2nd: 2d6 damage to up to two people, save negates.
- 3rd: Fireball is a 2x2 space thing that does 3d6 fire, save for half. Lightning bolt is 3d6 but cannot hit more than 3 people at once.
- 4th: Ice storm is a 3x3 area that does 3d6 cold, save half.
- 5th: Cone Of Cold: 3d8 cold, save half.
- 6th: Chain lightning: 3d8 electric to one target (no save), 3d8 electric to a second target who receives a save.
- etc etc
When using monsters, decide what kind of attack they should get:
- Pathetic: 1d4 to 1d6
- Weak: 1d8
- Strong: 2d6 to 2d8
- Overwhelming: 3d6 to 3d8
Saves for half for breath weapons, spells, etc. If it's too hard for the players, reduce the damage or attack or area of your critters, or use less of them. Or make raising dead relatively easy. You might want to use 5e-style damage resistance (half damage from that energy type) rather than using increasing numerical values. You will also want to cut down monster hit points. 1 hp for goblin-type swarms is always better than tracking HP. About 10-20 hit points for groups of human type foes. Solo monsters can have anywhere from 50-100 hp depending on how much of a slog you want the fight to be.
You probably think it is stupid to go through an entire d20 rulebook marking it up, but I promise it's much faster and more enjoyable than you'd expect. I enjoyed it, anyway. I did the above in like ten minutes with no intention of using it. I'm sure you can do better with a few hours and some consideration about your personal preferences.