Filling missing pixels in a contour

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This is for a OCR project where am doing the pre-processing of the image. One of the image, where some part of the text clear and visible but some are not.

Source Image

After pre processing

Here all the numbers are not clear.

Tried coloring all non black pixel to white, but some pixels in character where left behind which affected the OCR process. I want a solution where I can find all letters and fill any any missing pixels to make it OCR friendly.

All suggestions are welcome