There really is no way in Lightroom to improve the quality of a JPG from your camera to be equivalent of a RAW. Once it's a JPG, you only get a certain amount of quality in the image (which is less than had you shot the same image as RAW).
You can ENLARGE a JPG when you export, which really doesn't improve the quality, you are forcing Lightroom to interpolate pixels (in other words, make up pixels) so there is a loss of quality, although in some situations you might not be able to notice the difference.
A raw file doesn't give you any more resolution. The image is the same size as the JPEG. You cannot have anymore resolution natively than the original size of the image. Resolution of the image is not determined by the PPI setting. It is determined by the total number of pixels. An image that is 4000 x 6000 pixels can produce an image that is roughly 13 x 20" at 300 PPI. And that is true whether it is a raw or a JPEG image.
Actually, some cameras will take JPGs at lower resolution than the RAW or at the same resolution as the RAW. We don't know what camera options were selected, it could be that the JPG was taken at the smallest size in which case the resolution is lower than the RAW.