I have seen the poem title 君死にたまふことなかれ spelled 君死にたもうこと勿れ. I'm assuming given the poet's reputation, that まふ is some antiquated word analogous to the modern もう. What is it's use and meaning?
I would say this is a problem of both "the old grammar/words" and "the old orthography". A spelling reform (such as the German orthography reform of 1996) and archaic word usage are two different things, although they are closely related. Technically speaking, you can rewrite today's news articles using the old orthography.
たまふ in 君死にたまふことなかれ is an honorific auxiliary verb in archaic Japanese. Modern Japanese speakers don't usually use this word anymore regardless of its spelling. An equivalent in modern Japanese is ～なさる or お～になる. It's 給ふ in kanji using the old orthography, たまふ in hiragana using the old orthography, and たもう in hiragana using the current orthography. Modern monolingual dictionaries list this word as たまう and たもう even though old people did not actually spell it like this. To be clear, the adverb もう meaning "already" is not relevant at all.
It's pronounced like タモー ("ta-moh") regardless of the spelling.
Similarly, mono no aware is written as もののあはれ in the old kana orthography and もののあわれ in the modern kana orthography.
You are encountering here the difference between modern spelling and the spelling used prior to WWII.
まふ is the old spelling for もう
You can find other examples here.
I cannot explain the meaning of the old Japanese. I am unable to decipher its grammar. But in this situation, this is a case of differing orthography from an earlier period.