You have fenders, so that makes it more complicated.
Do you have a front brake? If you don't, get that first. Contact Venic Motor Bikes on this forum. Ask if their front disc brake adaptor will bolt onto your front fork. If it does, the 203mm BB7 front disc assembly would be the absolute best front brake addition you could do.
If their adaptor won't bolt on, you could shop around for a used fork that's fit your bike. Now would be a good time to install a front suspension fork, which would also have a front disc brake mount.
The front disc brake alone might be all you'd need, along with your coaster brake.
The front suspension fork is also a good addition, and easy to install.