You could capture a Growlithe as a fire type early on. It could help you beat the Ecruteak Gym with it's Bite. However, it will be a long while before you could evolve it. Alternatively, Magmar (burnt tower, lower level) is also available as soon as you reach Ecruteak. It's pretty rare, but the long search pays off.
You might want an Ice-type or Rock-type attack to help you against Dragons. Alternatively, you could just use Wooper and teach it Ice Punch or Rollout (only effective against Dragon/Flying though). I'd still highly recommend a different pokemon.
Your current team could welcome a few others such as:
- an Electric Type like Ampharos or Magneton
- an Ice Type like Swinub or Jynx
- a Psychic Type like Jynx (which is part Ice, too) or Xatu
If you can't keep your levels high enough for the League, just include an Ice type and you should easily get through. Leave the last slot for an HM slave.