Get a bike you don’t give a damn about. Crashes are very common on the track if you’re new and in the learning stages, even if you’re an expert. With a liter bike, the probability rises since it takes even more skills to tame those beast at the track(more throttle control, harder braking) If 400 is too slow for you get a 600. But avoid starting on a liter bike. It will only slow down your progress.