Level Transition (and reset enemies and weapons/ammo)

In Half-Life, is it possible for when the player goes through a level transition to have all of the enemies, weapons, and ammo to respawn? Usually when the player passes through a level transition all of the previously defeated enemies stay dead, and the collected weapons and ammo are no longer there, so is it possible to make everything respawn again?

I am wanting to make some stage levels that can be replayed, but I am not sure if it is possible or how to do it if it is possible. Thanks!