Because the Vespa style motor is rotary induction, you can't effect change to the intake without splitting the cases.
I wouldn't run without a head gasket unless you plan on lapping the cylinder and head mating surfaces on something that has less than a .01" deviance per foot.
If you don't have a granite slab/thick piece of plate glass/honing stone, than don't run without a gasket.
In terms of building your own expansion chamber, I doubt you will be more successful than just buying one, and you'll probably be happier and not spend as much.
The Tecno circuit pipe is super affordable and gives a nice boost, although not the best.