The left one of these can probably both be a classic and a modern styled shirt. But since they were released together they get on the same page. 

The Sega Shop website is bringing Knuckles tees to the table in 2018. They are in adults sizes only, but are 25 dollars each.
The first is an all red tee with simple white crescent shape on the chest so that you can be like Knuckles too! This is a fun cosplay / subtle design shirt that only a Sonic fan would recognize. It's a good idea.

It also has a counterpart sweat shirt over on Winter Wear. The 2nd is a gray tee with silly slogan. It uses the nwe forward facing classic Knuckles stock art & has "School of hard Knuccks" written in a circle around him with a 1994 year plate below his feet. It's a wacky play off of 'school of hard knocks' phrase.