Changed all private variables, properties and methods to protected. Changed all methods methods to virtual. Showing only lines that were changed.

CharacterStat_Extendable.cs

protected bool isDirty = true;
protected float lastBaseValue;
protected float _value;
public virtual float Value {}

protected readonly List<StatModifier> statModifiers;

public virtual void AddModifier(StatModifier mod);
public virtual bool RemoveModifier(StatModifier mod);
public virtual bool RemoveAllModifiersFromSource(object source);

protected virtual int CompareModifierOrder(StatModifier a, StatModifier b);
protected virtual float CalculateFinalValue();