It appears that your Sport came with a "D" load range tire which is an 8 ply/high psi tire..., which is a nice tire spec for the Sport.
In the 185/80R13 size Maxxis only makes a "C" range (6ply) tire in the size tire you presently have, according to the following Maxxis chart: http://www.maxxis.com/Industrial/Tra...ST-Radial.aspx
IMO, I would not drop down to a "C" just to get a 13" Maxxis tire.
Going to a 14" wheel with a Maxxis still only provides a "C" range tire (if clearances allow) with a little more load capacity..., but 2 less ply and a lower psi.
Assuming that load weights are met, I tend to lean toward the higher ply/higher psi combination in tires for the firmer sidewall..., but that's just me.
Give Maxxis a call, maybe they do make a "D" tire for a 13" rim but don't have it listed on the chart.