Rotax runs pretty high minimum weights, so you might look around to see if any of the local clubs allow you to run at something less. For example, the IKF (International Karting Federation) and TaG USA weights are about 15-20 pounds less than Rotax Challenge. You might look for a local track/series that uses those weights. Since you are first year and (presumably) not looking to qualify for the Rotax nationals or internationals, attending races certified for the Rotax Challenge Series probably isn’t a priority.
The good news about your situation is that if you run a race where they’re using those lower weights, it’s a heck of a lot easier to lose weight by pulling off ballast than by dieting.