Is this a hydrostatic (variable speed) model or gear drive 2/3 speed model? The formal model name is "HR215HXA" or "HR216SXA" The serial number indicates this is probably a K1 model (second version built). Honda does not offer any rebuilt or alternative transmissions for this mower. You might get lucky and find a used one (Craigslist) with a bad engine / good transmission.
A whole new trans for the hydrostatic drive is Honda Part Number 20001-VD6-305, list price $294.61
A whole new trans for the gear drive is Honda Part Number 2001-VD6-J04, list price $313.42
If you can troubleshoot / inspect it and find the fault, some individual internal parts are available for both models. If you dig that deep, a shop manual (paper copies) is strongly recommended. Honda sells them on eBay and Amazon, about $37, free shipping.