The origins mod supports power types which modify minecraft attributes. These are origins:attribute and origins:conditioned_attribute.
Using attributes, you can modify a player's maximum health, knockback resistance, movement speed, attack damage, and a few other things. For example, the attribute "generic.max_health" for example will increase your maximum health. Just put the attribute id (like "generic.max_health") you want into the "attribute" field, choose an "operation", and then provide a value to apply the operation on. Test out different values and operations (or check the wiki) to see what they do in-game.
Note that by default the origins:attribute power type does not support a condition. If you use one, it will be ignored. Use origins:conditioned_attribute instead if that's what you want.
Also consider using Pehkui if you want to modify something that isn't a default attribute modifier.