Short answer: Scout, detect the roach build before the Roach Warren, and change your build as necessary. Alternatively, start with marine/marauder and change to your hellion build if you detect a non-roach build.
Long answer: Let's assume you detected the Roach Warren 10 seconds after it went up.
If you detect the roach warren early enough, you can immediately throw a tech lab up (25 seconds) on your Barracks and build one marauder (30 seconds) in the same amount of time it takes the Zerg player to build their Roach Warren (55 seconds).
- 0: Roach Warren goes up.
- 10: Roach Warren detected.
- 15: Barracks starts building a tech lab; 2nd Barracks starts to be built.
- 40: Tech lab complete; Marauder 1 started.
- 55: Roach Warren complete, Roach 1-3 started.
- 70: Marauder 1, new, larva, Roach 4 started.
- 82: Roach 1-3.
- 95: Barracks 2, tech lab started.
- 100: Marauder 2. Marauder 3 started.
- 102: Roach 4.
- 120: Tech lab complete; Marauder #4 started.
- 125: Marauder 3. Marauder 5 started.
If at this point you see 4 roaches on your doorstep, you'll have 3 marauders complete (assuming a rush takes 23 seconds from when their 4th roach came out).
- 4 roaches = 32 DPS, 580 HP
- 3 marauders = 40 DPS, 375 HP
Statistically-speaking, the roaches will win without stim. If you researched stim, it'll be very close. If you had a bunker, you'd be fine.
Now, if they wait until they have 6 roaches:
- 85: Roach 5 started.
- 100: Roach 6 started.
- 112: Roach 5 completed.
- 127: Roach 6 completed.
- 150: Marauder 4 completed.
- 155: Marauder 5 completed.
Given the rush of 23 seconds, 6 roaches would appear in between your 4th and 5th marauders completing.
- 6 roaches = 48 DPS, 870 HP
- 5 marauders = 66.65 DPS, 626 HP
You're slightly better off. Without stim, it's close. With stim, you win. With a bunker or two, you are fine.
Note that this is all theory. Going for a hellion rush and having to bail out on it is not good news. Wasted time, resources, and a sense of panic can cause you to falter in this position. Even if you're able to stick to the example above, you won't be in an excellent position. You'll be forced into trying to scout to detect when the Zerg player changes their build to counter it and you'll be in a position where you're constantly reacting rather than being proactive and causing the Zerg player to react.
The best solution is to detect the roach build before the Roach Warren drops and get a head start.
If you watch a standard roach opener, you'll notice that rather than expanding after their 13/14 pool, the Zerg player is forced to save the minerals, wait until 18 to throw out the roach warren, and build up minerals for roaches rather than an expansion. When you don't see that expansion after the pool, it's a good sign that Zerg is going with a roach build. This is especially true if you do not see an upgrade happening at the spawning pool right after it is done. The upgrade happening at the spawning pool, in combination with an extractor being built early, usually hints at a baneling bust build instead.
If you swap over early, you'll be in a much better position than you would be if you waited for the Roach Warren to start building. You'll be proactive, the Zerg player will be reacting, and you'll have the upper-hand.