You can do a lot to make a GSR look like an Evo but it takes a lot of mucking around that you might as well have bought an Evo in the first place lol.
But there's a major difference you cant replicate on a GSR because it involves something you cant bolt in or remove and spot weld back into position - the length of the chassis rails!!!
When you remove the front bar, look at how far the chassis rail sits out from under the driver's headlight.
I took a pic about 3 years ago comparing EVO vs GSR in that particular location and stumbled on the difference by accident.
Not at home at the moment, but will post up a pic later to show you.
Edit - here it is