I'm in a similar position to you mate, I'd like a mighty / super mighty to drive in Assetto Corsa. At the moment I think the closest you will find is the 64 Cooper appendix K car, although having spent a few hours driving it I'm pretty sure that it doesn't handle anything like a miglia, or a mighty and the only suspension / steering settings available are ride height and camber so there's not much scope to change the way it handles.
I'm currently researching in to developing a custom car but it's pretty complicated. if it's possible to I will probably start using the 64 Cooper as a base and adjust the engine and brake performance and add in more settings for the suspension / steering.
Ideally I would like to find a mk3 onwards body for a more accurate representation, there are a few 3d models available but none I've found look right, then there is the complication of adding a miglia style wheel arch & removing the rear valance to complete the look
I'll let you know if I get anywhere with it. I may need some input from you regarding the setup on a mig.