Hard AI, from what I can tell, actually gets 1.5x income, not 2x income.

There is only one setting you can change that affects AI difficulty levels: scalar bonuses to diplomatic points for diplomatic victory, adjustable as aiDiplomaticPointBonusScalar under playerDiplomacyAIDef in gameplay.constants. There are no other ways to modify the differences between AI difficulty levels.

However, there are ways to make AI-only techs, so if the mod will be for personal use only, you can create a 0-cost, first tier tech that gives an income penalty. The AI will usually tend to research all tier 0 techs (so one lab required) regardless of preset, especially if said techs cost nothing. As a human player, you can either choose to not research the tech yourself, or you can place the tech in an out-of-bounds location: the AI will still research the tech even if you cannot click on it in the UI. AI-only bonuses and penalties in other mods are all done this way (effects are tied to a 0-cost, first tier tech that the AI will always research within the first few minutes of the game).