I have been wanting to do this myself for years.
A change where I don't feel I have the Expertise to do on my own.
I can strip the bike of all its body parts, do a lot of the maintenance, but not something like this. I have had conversations with a few subject matter experts about me stripping the ABS off my Bike and deliver it to them Naked for the update, and I will re-assemble the Bike to save on Labor.
I put my 2009 MP3/250 in the Category of a "Collector Bike", since you can't buy them anymore, and it has by far the least amount of problems as the MP3/500's. The Engine from the GTS/GTV "AKA Quasar", and other Bikes sold by Vespa is an outstanding engine, but it just lacks power for the heavy MP3/250.
Please keep us informed on your progress if you go this route. You are basically making a MP3/300 which I think it will be a better bike especially if you ride in Hilly Roads as I do in the North Georgia Mountains.
You will also need a Performance Exhaust with the items you have listed which is getting harder to find, but I think Malossi still makes the slip one but they are expensive. I run the LeoVince and have a New Spare as I knew the supply of them were drying up over the years.
Keith,
Marietta, GA