you should try and pick up a 20/17 or 20/20 carb. if you can get the 17, i would jet it like a stock VBB. If you can get the 20/20 (a better option overall, should you want to run a pipe or higher revving cylinder later on) jet it like PX150E (1984 version).
Keep in mind, these are just starting points. You'll still have to actually tune your carb by doing proper plug chops.
Keeping your original 20/15 carb and running a 150 jug, isn't impossible, but you'll have to do a lot more leg work since it wasn't ever a documented combination by piaggio. If I had to guess, i'd start with the VBB jetting, and keep working my way leaner until it doesn't bog to much. Again, all of this is speculation.
What part of the world are you in?