I'm a fan of flamenco music.
I'm a football fan.
With sports, you could say "a supporter of" but it sounds a little strange, unless you specify the team (I'm a Real Madrid supporter).
If you don't want to say "fan of", you could simply say "I like..."
We don't say "keen of", we say "keen on". I'm really keen on football, for example. However, I don't think "keen on" is any more formal or informal than "a fan of".
Student or Learner