Is "it is more difficult and nobler an enterprise" ungrammatical? I don't think it is grammatical, and while I'm not going to claim that I know through and through of it, I believe that this part of The Cambridge Grammar of the English Language (CGEL) by Rodney Huddleston and Geoffrey K. Pullum, page 435, can explain this problem.
AdjPs introduced by the degree adverbs more and less
[7] a. This is [more serious a problem than the other].
[7] b. This is [a more serious problem than the other].
With AdjPs of this form, there is alternation between the external position [a] and the internal [b]. Again, the external position is available only in combination with a -- compare the bare NP These are [more serious problems than the others]. Note also that inflectional comparatives are restricted to internal position: Kim is a better player than Pat, not *Kim is better a player than Pat.
Similarly to *Kim is better a player than Pat, which is ungrammatical, It is more difficult and nobler an enterprise is ungrammatical, because "inflectional comparatives are restricted to internal position" as mentioned in CGEL.
Selected Writings by Sir Philip Sidney, Richard Dutton "Even for his near contemporaries, then, the Sidney myth could be hard to swallow. Today, the chief danger is that it distorts our understanding of the man and his times and so makes more difficult a proper appreciation of his writings."
– Makoto Kato May 24 '16 at 07:12