The only way how to convert older WordPerfect documents (for example .w51) to MS Word formats (.doc, .docx) is probably to get access to some previous version of Corel WordPerfect that still supports W51 format and save it to some widely supported format, for example RTF (Rich Text).
Once you have your document saved as .rtf file, you can easily convert it to docx in almost every modern word processor, including MS Word.