We`ll, I`m not such a pro on giving evs to pokemon xD
There are many possibilities
- You could max.ev it`s Defense (to protect your Magmortar from a death caused by an earthquake/stone edge).
- You could max.ev it`s Sp.Def (Magmortar isn`t very fast - before you`ll be able to use Sunny Day, the opponent can use Hydro Pump on you. Otherwise if the opponent has got a leaf/bug/ice pokemon and if you call Magmortar, then the opponent has a problem. He`ll may get forced to switch his pokemon. When his water pokemon comes out, then you`ll use Sunny Day without taking a damage by a water attack - you can`t attack in the same round as the pokemon was called)
- I wouldn`t worry that much about it`s Sp.Atk. It`s high, but you could also ev.max it.
It`s up to you :)