I saw a picture of one with 20's. What mods are needed if any? Will bigger wheels fit?

 

20's are PUSHING it he's running rubber bands for tires , steering radius blows, fenders need to be rolled , can't be lowered ...
20's the biggest unless you raise it ...

 

look at the tire diameter that you would run. anything that is larger will raise the car. one in my home has larger tires due to larger wheels. looks ridiculous. takes away all the sleekness when you go up. just my opinion, but something to be aware of.