I used to have a similar B210 at my place couple of years back. Its chassis is very soft, and not designed to take much abuse from a powerful motor such as a 4AGE. And most importantly 20v itself is not a very good motor itself. So in my opinion, its better to drive it on the original A12.
But still if you want to go for a swap/modification, then kindly search thread by SER_GTR for a 20v conversion. He swapped a 20v in a sprinter. Or search just 20v itself, Pakwheels is full of 20v information.
Thnxx..