We did the site conversion in 2 weeks (2 extra weeks for our own additions). We are now cleaning the code -- porting include files to master files + ASCX controls, implementing caching object, creating OR mapping using NHibernate, real .NET classes (yay), TinyMCE, switch to C#, etc.
To let you know, the ASP.NET site is about the same speed as the ASP site. You should seriously think about porting to .NET because it would only take you about 2 months to do the port with 2 people (not 18 months) and will allow you to do TONS of better things with WWF right off the back.
In fact, I believe Mono will mostly work with ASP2ASPX (although I'm sure there is some code touch-up).
Sincerely,
Michael