Swords and Wizardry is meant to be modified to your liking. Here's the set of house rules that I use in my current Swords and Wizardry campaign.
Roll 3d6 for stats and place them wherever you like.
You may choose to replace any one stat with a 16.
Only fighters get more than +1/+1 damage due to STR.
Clerics get a bonus first level spell if they have a WIS >15.
All characters receive maximum HP at first level.
At higher levels characters roll their HD to see how many HP they gain. If the roll is less than 1/2 the maximum then they gain 1/2 the maximum.
Add level to backstab damage.
Assassins gain +2 bonus on save vs poison.
Clerics may cast CLW once per day in addition to their normal spells.
Clerics may scribe scrolls for 100 gp per level of the spell.
Clerics must be Lawful or Chaotic, a neutral cleric is a druid.
Clerics must choose a deity.
Clerics receive bonus spells from their deity.
Str bonus applies to both HtH and ranged for fighters.
Magic users can cast read magic and detect magic once per day in addition to their normal spells.
Magic Users may scribe scrolls for 100 gp per level of the spell.
Dwarves may be fighter/thieves, fighter/clerics or a thief.
Elves may be Fighter/Magic-Users, Thieves, or Fighter/Magic-User/Thieves.
Half Elves may be Fighter/Magic Users, Fighter/Magic User/Clerics, Thieves, and Fighter/Magic-User/Thieves.
Halflings may be Fighters, Thieves, or Fighter/Thieves.
The ascending armor class system is used.
Any damage from a single attack can be ignored by being absorbed by a shield, including a critical hit. When used this way, the shield immediately becomes unusable, shattered by the blow. Magic shields, on the other hand, don’t get destroyed, but are unusable until repaired by an armor smith.
If you are wearing a helmet and take a critical hit that involves the head, you may choose to ignore it but the helm is ruined.
Characters, major NPCs and big monsters can inflict critical hits. On a roll of 20 roll on the appropriate Crit table. The crit tables from Dungeon Crawl Classics are used.
Clerics,Thieves, Monks, Druids use Table 2.
Thieves and Assassins automatically crit on a backstab.
Monsters use the appropriate monster crit table. Roll die type according to HD.
Instead of having charges, magic items now have a charge die assigned to them. Every time you use the item roll the charge die, if you roll a 1 the charge die drops to the next lower die. If you roll a 1 on the D4 then your item is out of charges. For example, if an item has d8 charge die, then you would roll 1d8 each time you use the item. If you roll a 1 on the D8, the items charge die drops to a D6. The Die order is D30 D20 D12 D10 D8 D6 D4.
I also tend to hand out magic items and spells from AD&D that aren't in the S&W complete book. I use the rules from the original AD&D books or the OSRIC book. The OSRIC pocket SRD book is great for looking stuff up during the game.
...and I'm stealing many of your ideas. Thank you, fellow gamer.