One great Chibi Kanda Yuu fanfic is 'The Adventures of Chibi Kanda'. It's really cute as it shows Kanda in a more child - like and innocent state. The author does a wonderful job of portraying his character traits in a chibi form.
To draw manga like Yuu Watase, you need to understand her use of composition, panel layout, and story-telling techniques. Also, focus on her unique way of shading and adding details. It takes a lot of practice and patience.
A good Chibi Kanda Yuu fanfic should first of all capture the essence of Kanda Yuu's character. Even in chibi form, his key traits like his determination and his slightly prickly personality should be there. For example, if he's in a situation where he has to fight, he should still show that tenacity.
There is a bit of a love story in Yu Yu Hakusho. It's not the main driving force of the show, but it does contribute to the character development and adds some emotional complexity to the narrative.
I'm not sure specifically which 'Mika x Yuu lemon fanfiction' is good. You could try searching on some popular fanfiction websites like Archive of Our Own. There might be a variety of works there that match what you're looking for.