You already got one step. Its called the deck. Just when you park the mower lower the deck onto the gauge wheels. This is so when step on the deck it don't moves. I am 250 and most decks can handle my weight. Also stepping from the side is a little more stable as can steady yourself with one of the lap bars, just don't put your full weight on the lap bar.
If an additional step is needed then a metal fabricator should have no problem designing one.