Well, NEWGAME=1: trigger the running of a new game (typically from title page) but not just the level, a whole new game I think.
Otherwise you could use PLRMOVETO to transport the player back to the starting point of the level, of course it would not reset any of the entities/characters etc.
The only other thing I can think of is to set a checkpoint somewhere and kill the player (PLRADDHEALTH=-500) so he returns to the checkpoint, again that does not reset the entities/characters etc.
Like Filya said, theres only 2 ways. Thats all I can think of. There is no RESTARTLEVEL command.
Best.
I'm sorry, my answers are limited. You must ask the right question.