Porting Sketches iOS App to Mac OS

We want to port one of our flagship app on Mac: Tayasui Sketches
[URL visible pour les membres Pro]

The drawing engine is already ported, all we need is the GUI around it, which will be very close to what we have today.

You would benefit from our code so that that you perfectly know how it behave and you'll also have a basic working App with the engine working on Mac.

We'll provide a specs on what will be in the App and how it behaves but let's say it will mimic the iOS App for 90%.
Yet file management will be deferred to Mac OS and Sharing will be absent too.

We want a good estimate of the time to port the App and a fix budget (+- 20% will be ok in the end)

thanks in advance,
yann.