import nltk
from nltk.tokenize.moses import MosesDetokenizer

my_dict = {'abc': 'aBc'}  # put your special dict here

# split the sentence in words
words = nltk.word_tokenize(raw_sentence)

# replace those that map in the dict
new_words = [my_dict.get(word.lower(), word) for word in words]

# now we construct it back to sentence
detokenizer = MosesDetokenizer()
detokenizer.detokenize(new_words, return_str=True)