Just one thought from me:
I've quite recently made a demo project in cocos2d-x, where user can draw numbers in a rectangle. Then I convert image to open cv format and using SVM classifier from here: https://www.simplicity.be/article/recognizing-handwritten-digits/ it tries to recognize written number. From my experience it works pretty bad. There are few numbers, which are corrently recognized in 90% of times, but some are more like 20% or even less.
So, if you want to include characters, and what's also making harder to recognize, whole words (or sentences) I don't think this could a good method.
If you'll find some more effective ocr technology let me know