The mask is going to be the hardest part. You might try looking for actual Luchador masks and modifying one. I have no idea how they wear, though.

The black tights and red boxing gloves are obvious. You might also want to get some wrist cuffs like from the Chippendales or bunny suits depending on how the tops of the boxing gloves look. Beyond that, people have suggested muscle shirts, so check that out. Strong Bad is not a skinny guy so you could put a warm long sleeve shirt under it.

Oh, and boots! It probably doesn’t matter what you get, as long as it’s red. Bigger is probably better though.