Parts catalogues here

[forchetto]

If the crank assembly is like the Qingqi, then they won't be able to do it. The chances are that it is of the type that comes with one-piece non-detachable conrods. That is the crankshaft and conrod has ball or roller bearings and the rod is assembled on a press and supplied as one piece. The balancing machine could not cope with a loose conrod.
Dynamic balancing can only be done on engines where the rods are two-piece with plain bearings, and the crank is one-piece like a car and the rods can be taken off the crank.

In this exploded view you can see what I mean, the whole thing is pressed together under enormous pressure:

[txm123]

How do you call the part number 4 in the drawing?

[forchetto]

Crank pin. This is the bit that joins the two flywheels parts 5 and 6 together. The one-piece conrod and its roller bearing are assembled before the bits are joined together with a hydraulic press.

[txm123]

How can you say that is a balancing shaft or not?what do I've to look for
?holes ?

[forchetto]

Don't know on these bikes. In the old British bikes there were holes half-drilled around the flywheels where they removed some metal to balance them.

Here's a flywheel with balance holes in it:

[txm123]

Have you ever tryed to weld the rear fork or make some kind of clamp in order to put a rear stand to the second passenger in bikes like my,that they don't bring one?

It very annoying to the second passenger to have his/her legs hanging...

[forchetto]

Most bikes have a sub-frame to carry the passenger footpegs. A sort of "V" shaped loop of tubing attached to the chassis. You can also just weld or bolt pegs to the swingarm, some bikes did come like that, but it's a bit less comfortable for the passenger as the legs go up and down with the suspension.

Don't drill the swingarm to put footpegs on it as it will weaken it. Instead there are clamp-on footpegs like these on this page.

[txm123]

This was exactly, what I was thinking! Obviously someone has invented it already!I think I'll try to find a kit like this one in the market or I'll try to build one for me.
Being a bit technical,I wonder if this type of clamps don't tend to bend the swingarm I mean wrench it as when you step in the foot pegs the clamp tryes to spin,and so the swingarm...?
What do you think? Maybe is not as critical as It sound?

[forchetto]

I think it'll be OK. At least the swing arm on mine is a very strong alloy extrusion that looks strong enough for a bridge.

[CrazyCarl]

Any good rectangular cross section swingarm should be okay. It's not like the passenger will be standing on the pegs or anything...I REALLY hope not anyway.

Actually, it doesn't even really sounds like a good idea in general but I guess a place to put your feet is better than none.

CC