Update: for an example of an adaptation of the basic principles of XD20, see this system, presently nameless, designed to run simple dungeon and wilderness fantasy adventures with any source at hand.
Character CreationXD20 rules recommend deducting a roll from the maximum to generate stats, but there's no reason, since lower stats are better, not to roll and add to the minimum, or experiment with different rolls. Rolling 6 + 1d8 gives a wide spread for stats, but 2d6+3 or 6+1d6 give more focus and control.
TAC, PSYCH, and WAH map roughly to physical, mental, and spiritual (maybe just luck) abilities. Then there’s Health. These are easy to rename or reconfigure for different games. TAC (Toughness and Constitution), INT (Intellect and Training), PWR (Presence, Will, and Resolve), and Stamina for instance.
Health: In XD20 the weaker the character the lower the character's Health stat, since Health is based on a combination of the character's lowest scores. But Health, unlike stats, is better the higher it is. There's a nod in the rules towards game balance, but why should a mighty warrior tend to have worse health than an inept hedge-wizard? Instead, Health should start at zero (or 2 or 4 for constitutionally weaker characters with the Mystic/Magical type). Health would then go up (bad) as hits accumulate, and characters with more than 20 points against Health would be knocked out or killed.
This means that current Health can also be used for rolls if the character's constitution or fatigue is tested.
CombatXD20 is vague about combat damage, but effects, and hence weapon damage, are decided by a D20 roll. Bear in mind that even a dagger between plates of armour can kill. But to estimate damage, assign about 6 points for a small weapon or a light hit, 8 for a medium hit, and 10 points or more for a big hit. Pull back for adventure games and push for gritty games. Or just adapt the damage dice from the equipment table in any D&D retroclone.
A high success (based on the difference from the target) should nudge up a low effect roll.
To integrate parries, dodges, tricks and other multiple actions and reactions in combat, each additional roll might be allowed with a -4 penalty.
In the end, cut to the chase for combat: assume that weapons and armour are already integrated into TAC and Level, and only deal out adjustments for major tactical differences or special cases.