I spend time tying to get pictures taken with modern kit to look like this so this is a different problem for me
Making any real improvement is way beyond my ability and maybe can't be done but you could take a look at the various options available in Nik Filters Color Effex Pro. I don't know if Nik Filters will work with your processing software but as it's a free download it'll only cost you time to see if you can get it to work and see if you can get a look you like.
If you want to give it a go here it is...
https://www.google.com/nikcollection/
Good luck.