#1 - I would have asked the previous owner what he was mixing it at, and go from there.
#2 - There is a screw on the side of the case that says "oil fill", take that out and fill with oil untill it comes out of that hole, I usually buy a quart, and have plenty left over.
#3 - You could put a pipe on it for better acceleration, but if you want more speed you will have to consider gear changes, and tire sizes, just dont gear it too much, or you will have a very under powered machine.
1985 TRI Z
1985 ATC 250R
2004 YFZ 450