A d100 system is simple enough to convert to The Fade Tabletop 2e as part of it already uses the built-in d10 system, and when rolling d100s in person the dice needn't be swapped out (as d100s are typically rolled by using 2d10).
Characters stats remain unchanged in a d100 game, keeping to the 1 - 10 formula as per the core system.
When referencing a Skill, each one has a percentage chance the character must beat in order to succeed at the skill. Thus, the character's aptitude and DC are linked together.
Skills start out at a percentage equal to the stat associated with it and multiplied by 5 (for example, a Strength of 8 means Strength skills start at a 40% success rate).
Trained skills increase their percentage of success by 2% for each level the character has.
Untrained skills increase their percentage of success by 1% for every other level the character owns (or in other words, half level).
Due to the way rolling works, the "DCs" of checks have now become percentage numbers that the character must roll against and are largely determined by the other character's skill. To supplement this fact however, characters being attacked have more of a chance to get away from attacks.
Remove all Defenses.
Instead, characters gain two uses of Dodging/Parrying for Defense/Avoid, and one use of "dodging" for Bodily Defense and Mental Defense. When doing this, roll a 1d10 + (Stat) or in the case of Parrying, 1d10 + (Attacking Stat).
The result of the Dodge/Parry reduces the attacking character's % chance of success. (Example, rolling a 1d10+4 and getting a 9 against an opponent rolling a 86% chance attack roll - the dodging reduces the chance to 77%). This can be used after the attack roll is determined as a way to potentially negate the attack, similar to normal dodging rules.
Standard rolls in the system use the similar rules above used with Skills, except for when it comes to adding to the percentage bonus.
Attacking with a weapon you're proficient with (casting spells also applies): +25% to the Target Number (TN)
Using a class ability which requires some sort of roll not linked to a stat: 45% + Double Class Level
Miscellaneous bonuses (for example from Feats) add to the percentage chance of rolls. They increase the percentage chance by 2% per rolled die that would be added and 5% per kept die that would be added.
Flat bonuses that would otherwise add to a roll add to the percentage chance without being modified, for example a +5 in the game would become +5%.
Natural 1 is a critical hit, Natural 100 is a critical failure.
Damage in the d10 variant of the system is instead handled with either d5s or d10s. These are added together and totaled unlike the "keep" system which only total the (x) highest results.
1-4 keep dice rolls damage as d5s, and 5+ rolls damage as 10s.
The amount of dice rolled for damage is equal to half the regular amount of rolled dice + 1. For example, 8k4 damage would become 5d5 worth of damage. The stat that would normally be used for damage adds to the rolled dice, and adds half the stat as a flat bonus to the damage.
Effects which increase damage modify whether d5s or d10s are used (see above) if they do not add a flat bonus.