Some easy references that would

Obviously...the Creed shorts that Rocky ends up wearing to fight Drago...

https://images-na.ssl-images-amazon.com/images/I/61H2JC1h%2ByL._UX679_.jpg

Probably hideous if they made the jersey match the shorts, but you could make the jersey just a wife beater or the old 6ers jerseys

http://www.nba.com/media/sixers/sixers_jersey_180.jpg

There was the original robe he wore that could've been used

https://images-na.ssl-images-amazon.com/images/I/51tPhvzCQBL._UX522_.jpg

The red with yellow trim and the meat advert = easy.

Or the black and yellow short/robe style

https://images-na.ssl-images-amazon.com/images/I/61moA56l9tL._UX679_.jpg

Simple

sweatsuit grey doesn't make me think of Rocky.