Winform desktop app and touch

I`ve a legacy winform desktop app that works perfectly with mouse and keyboard. It has some selfmade controls that involve the creation of threads and so on, for example, the longer a button is pushed the faster a number is incremented. The application also uses a win32 dll. Now, the client wants that aplication to be touch enabled and run it in a tablet, which also means resizing and rotation capabilyties.

My question is, which is the better way to get that aplication touch enabled and responsive design?

I can try to modify the existing winform, but I think it will be lot of work with poor result. I can also migrate to wpf and reuse the c# code, but I might have trouble with the keyboard, as I have not found a good way to show the keyboard and mantain the whole app on the screen. Or I can migrate to windows store app, but with the problem of that win32 dll, that I`m not sure it could be migrated.

The winform application is multilanguaje so creating a keyboard is not a valid option.

Any advice is welcome.

Thanks