Anatomical pathologist here. Same thing happened to me a while back (bought a cheap rib protector). You’ll need to get a bone scan to the affected area. X-rays are not very efficient at diagnosing minute fractures. I’ve had cases including mine where an x-ray showed an all clear, yet the patient was still complaining of pain. Performed a bone scan and impact fractures showed up not in just ribs but other areas as well, ie, costochondral junction.
There is nothing you can really do to speed up the healing process. Only thing you can do is keep away from physical activity. It’s important to wait and let it heal completely. Usually to be safe 2-3 months. It will feel much better after about a month but it’s very important not to get carried away and go back to karting. You can actually do more damage in just one stint and resetting the healing process.