Magmar
65 HP
95 Atk
57 Def
100 Sp. Atk
85 Sp. Def
93 Spd
Magmortar
75 HP
95 Atk
67 Def
125 Sp. Atk
95 Sp. Def
83 Spd
Magmar has 10 more in base speed, which may not seem like much, but it's right at the crucial line between 80 and 110 where every little bit helps. Magmortar on the other hand has 10 more in everything except attack. Special attack gets 25 more.
With Sem's set, you could potentially get rid of any of the physical attacks for Overheat as a secondary, special STAB. It'll catch anything that tries walling you off-guard, but will potentially only have one use [for stat lowering] and cost you a slight bit of coverage. The choice is yours.
And for a different moveset:
Magmar@Grip Claw
Timid Nature
Confuse Ray / Smokescreen
Lava Plume
Psychic
Fire Spin
Lava Plume has a 30% chance to burn and a power of 80, Flamethrower has 10% and power of 95. with Magmar's Defense I like the increased burn chance.
Confuse Ray for mostly obvious reasons
Psychic for coverage
Fire Spin to compound damage with a burn, should you get it, and prevent fleeing from confusion or lowered Accuracy. Also as somewhat of a filler.
Barrier, Toxic, Mach Punch, Cross Chop/Focus Blast, protect, Endure could make valid replacements, but you should definitely have confuse ray over smokescreen if you don't have Fire spin to prevent the switch.
I say endure is a possiblity because you have Flame body and confuse ray. holding out for just that little bit more while taking an attack at the same time could work out pretty well with Magmar.
Cross Chop, because there really isn't another special move for Magmar to use
Mach Punch for Priority, and because I despise Weavile >>
As for EVs, with this I'd say full speed, and whatever split you like between Defense and special attack.