You can use Corid for coccidiosis. For E. coli and other gram-negative bacteria. I use Baytril (enrofloxacin), but that's banned for use in poultry and you would have to order it.
You could try injectable oxytetracycline if Tractor Supply has it, but I'm not sure how effective that it. @speckledhen mentioned injectable penicillin and that might be worth trying too, though it doesn't treat E.coli.
- Baytril source - https://allbirdproducts.com/products/baytril-10
- Baytril ban info - http://www.cidrap.umn.edu/news-perspective/2005/07/fda-ban-enrofloxacin-use-poultry
May I suggest that you contact this veterinarian and see if they can see your hen or refer you to someone that can?
https://www.yellowpages.com/basalt-co/mip/liz-kremzier-ms-dvm-cva-478634174?lid=1001780199150
My packet of neomycin + doxycycline mentioned about ecoli don't know if neodox forte available there