In the Burn disc for DVD player dialog, click "Encoder settings", then increase the quality as high as it will go and increase the bit rate as high as you can, so long as your project still fits onto the DVD.
Why don`t you try out the video cleaning functions of the product. Also you can seperate Audio/Video and clean the sound. For example with denoiser and than with compression & brilliancer to master.....
For sure you cannot convert bad original quality into a good one, but there are many options to improve the look....