You do not actually HAVE to put them in lava to breed.

What you can do is:

1. Enable mating and hit with wyvern fire, however you will need to bring the a Fire Wyvern in from another map such as Scorched Earth, Ragnarok, Valguero or Ember Crystal Wyverns from Crystal Isles

2. Do the opposite of 1: Upload the Magmasaurs to another map with Wyverns, Such as the examples listed above.

3. Enable Mating, then blast them with a Flame Thrower.

4. Enable Mating. Cover a desired area with oil and make the Magmasaurs stand on said area, then light it up.

WARNING: If you hatch a Magmasaur egg in the bog biome, the rain will KILL the babies.

I hope the the above info was useful to anyone wanting to breed them outside of the volcano biome.