People use larger carbs made by Dellorto, thats the most common. Exhaust can be de restrcited a bit, but I dont think theres much of a point UNLESS you also run a larger carb.
If you are very mechanically inclined, porting costs virtually nothing but drill/dremel bits and time, and yields fantastic results.
With a 2 stroke, the best bolt on is expansion exhausts, but for a Moby, expect some custom work and a near $200 price tag for that, not to mention probably having to figure a way to get one out of Europe.
Theres every performance part you can imagine in Europe, but nothing incredibly cheap or easy to get.
If not just for performance though, the Dellorto is recommended as it is considered more reliable, more tunable, and easier to work with than a stock Gurtner. I use one, I like it.