BULLETS OVER BROADWAY: what would Woody have done?

I've got just one problem with this Woody Allen comedy about a struggling playwright in 1920s New York who's forced to cast a gangster's talentless girlfriend in his latest production to get it produced.
Woody's not in it.

He wrote it and directed it but chose to cast John Cusack in the part of playwright David Shayne even though it's clearly written for him.
How do I know that?
Because Cusack's performance is not so much an interpretation of the part as it is a very obvious impression of Woody Allen playing the part.
And therein lies the problem.
When a character is so blatantly written to be played by one particular actor I want to see that actor in the part, not another actor playing it exactly like the actor it was intended for. It's so frustrating when Allen is standing there, just a couple of feet away behind the camera. He could just have walked out onto the set and taken over, for cripes sake!
Cusack does a great impression of Allen but he's too young to be convincing as Allen.Conversely Allen is too old to play the part as written. A struggling 60 year old playwright seems a little unusual - surely anyone with half a brain would have given up the struggle well before reaching the three score mark. The character of Shayne would have to have been re-imagined for Allen to have played him.
But I would rather have had that than Cusack's impression. Because it is so patently Allen, Shayne doesn't ring true as a character. And that kind of drags down the entire film.
Well maybe not entirely. On the plus side, there's impressive performances from Dianne Wiest, in an Oscar winning turn as a Broadway grand dame who's seen better days, and Rob Reiner as a pretentious fellow playwright who prefers to cling to his principles than write a hit play and trades philosophical barbs with Shayne even as he's stealing his girlfriend.
By no means is BULLETS OVER BROADWAY a disaster but it could have been so much better with Woody in front of the camera as well as behind it.