Here's guide to tyre sizing. Quite a lot of people have gone to 90-series tyres. Opens up more options on different brands and models of tyres you can use.
https://faq.ninja250.org/wiki/Ride_h...ent_tire_sizes
Speed-rating is spec on how well tyre resists heat-buildup due to casing flex. All tyres will flex certain amount at contact patch based upon load and pressure. Higher speed ratings means tyre will stay cooler at any given speed. I suggest no lower than H-rating.