The widest you can go is 200mm. Unfortunately, the narrow profile of the rim squeezes the wider tire which creates a TALLER tire, rather than a WIDER tire. When on the rim, the 200mm tire cross-section is less than 1/2" wider than stock, which is not noticeable. Save the extra $$$ and just run the stock width.
There're no aftermarket options. If you find a slightly wider rim and pay to have it laced to the C50 hub, you will have clearance issues with the driveshaft housing. As it is, there is less than 1/4" of clearance on the stock rim with a 200mm tire. Any wider and the sidewall will rub.