90/90-18 will never fit on 17 inch rim size for this you would need to purchase 90/90-17
and rim can be changed but to fit would you need to buy new spokes as well
i would not recommend installation of 17inch rim it will decrease height from rear in my
opinion it will ruin fun of pridor as its height adds pleasure in ride but if you personally like
having rear little bit lower or smaller tyre at rear like cg125 than you can do it its possible
you would need to purchase cg 125 rim, spokes and tyre of size 3.00