>>18812218My idea is pretty close to yours. Back when I used to toy with hacking ROMs, I would always give Magcargo and Slugma stupidly high Special Defense stats.
His stats now are
50 HP
50 Atk
120 Def
80 SpA
80 SpD
30 Spe
I usually bumped that up to
50 HP
60 Atk
130 Def
100 SpA
200 SpD
30 Spe
That's an increase of 160 BP, though, so really, only an evolution could save Magcargo, sadly. 50 HP is too low, even with 200 base SpD. If Marcargo evolved, got a better ability, and had a massive spike in SpD and a moderate boost to HP and SpA, say, it would make for a nice niche Pokemon.