Rick,
Here's one, right here. The assembly is Lionel's 630-1771-150:
The electronics board at the far right is what you describe, the rest is the air whistle that it drives. So this one's not for the big, old pulmor-driven air whistle, but a smaller more efficient modern one; one that still sounds very good.
This isn't the solution you're probably looking for but it's in the ballpark.
Mike