Hey Guellao, Welcome to the forum.
A Jack Shaft is just a shaft that either transfers the drive from one side to the other, or is used to aid in gear reduction.You can see a pretty good example of one here that transfers the motor drive to the peddle side. Post# 5, Pictures 1 and 2
And yes, Being a BIG guy, you might want to consider a larger engine. I don't know the law in Brazil concerning motorized bicycles, but they CAN'T be as hard as the laws in the States.
And if you run into a guy named Ivan Mansurov, a friend of mine, getting drunk in a bar, Tell him RED says HI. He recently moved back to Brazil and I have not heard from him since.