You could certainly display your "clan badge" on a t-shirt without any problem, but you might want to re-think displaying the motto by itself. Here's why:

Let's say your chief's motto is "semper fidelis" -- always faithful -- and you print that on a t-shirt all by itself, no crest, no strap and buckle. People could be forgiven if they thought you were a Marine, or a member of the 21 other families that use Semper Fidelis as their motto. That's why it is best, to avoid confusion, to use the entire clansman's badge to show your allegiance to clan and chief.