It is also worth pointing out Joel says "don't rewrite" specifically in the context of a software company, where I strongly believe it is largely correct. In the context of a decades-old open source project the decision may be quite different for legitimate reasons.

I find it likely that you'd be better off performing an algorithm extraction and doing a true from-scratch rewrite than trying to incrementally translate TeX and LaTeX.

For the reader's convenience and reference: http://www.joelonsoftware.com/articles/fog0000000069.html