I would think that the very idea that you're considering adding something to stall
in your :fire: big fire :fire: deck should let you know that it's not really a viable option anymore. Sure, you could play Magcargo EX in a deck, and there are many ways to make it effective. However, it isn't going to replicate the speed of the old entei/cargo decks. In all my playing days, which are considerable, I've never seen anyone run anything to stall in a big fire deck, since they're all about speed: if you're looking for a titanic hit in the fifth or sixth round (thus requiring something to stall for awhile), there are better options out there than magcargo (with 80 hp) or Magcargo EX (which nets your opponent 2 prizes) - options that don't discard energy or aren't weak to the ultra-common water type. If you would like to run Magcargo EX, and are content with the knowledge that there's nothing in the current format which will net you the sheer speed of entei, then by all means, try some of these:
Harvest Bounty Venusaur
Sandstorm Elekid
The Aforementioned Blaziken
Holo Swampert (an interesting choice, since Magcargo EX discards basic energy to do its extra damage, rather than exclusively fire energy)
Energy Transing Sceptile (See Swampert for reasoning)
Oracle/Pokenav + Magcargo EX's first attack
Essentially, anything that allows you to attach energy more quickly to Magcargo will be your friend. However, you'll need to keep in mind that its speed cannot match that of the old entei/cargo decks. Recovery after your first big hit will be much more difficult, you will be vulnerable to the tune of two prizes every time you leave cargo up there, and your setup time will be significantly longer; I've lost to a number of entei/cargo decks over the years, in big tournaments and elsewhere, but never, ever did I lose to one that failed to set up by the first or second turn. If there wasn't a slugma and entei on the board at the end of my opponent's first turn, I felt pretty secure with my chances. By the same token, if my opponent is howling at the end of their first turn, and slugma is on the board, I'm nervous, no matter what I'm running. You won't have that speed, and there will be stuff out there faster than your cargo -ex...stuff that isn't -ex....stuff that is water...stuff that recovers better....
Good luck, though.
Cheers,