According to your two problems:

1. performing spell checks does not give entire string as an output

2. sometimes repeats the phrases of the given review

I think you can adjust two arguments ( max_length, no_repeat_ngram_size ) for model.generate() to improve two problems:

1. Enlarge max_length size to solve problem-1.

2. Add no_repeat_ngram_size argument to reduce problem-2 error.