So your repentant assassin would lose his sneak attack
, death attack, poison use, save bonus against poison
, uncanny dodge
, improved uncanny dodge
, and hide in plain sight
class features, as well as his assassin spellcasting and any weapon and armor proficiencies
gained from the class. He’d keep the skill ranks he bought with his assassin levels, as well as the hit points, base attack, and base save bonuses gained from those class levels. He also couldn’t gain any more assassin levels until his alignment returned to evil (at which point he’d also regain the various features he lost when his alignment changed to non-evil).