I'm racking my brain at how to say this nicely but, those looking at posts for answers need to realize that (as in this case) some of the folks shoot themselves in the foot and then want to complain.

Unfortunately, you were using a ZC version released over eight years ago using a PHP version that was End Of Life over six years ago.

And then, that nasty host had the audacity to upgrade to PHP 5.6 which was EOL two years ago. They could have updated to PHP 7.2 BUT, that was EOL a month ago. Even PHP 7.3 will go EOL in a year. The good news is that you can get 1.5.7b going with PHP 5.6. However, I'd be concerned about when the host is going to be able to provide for your newer needs.

It depends on the terms and conditions of your host as to whether or not they were required to notify you of the changes in hosting. We keep track of who's using what and would never drop their coverage without notifying them ahead of time. Fortunately, other features of our servers allow us to do this without losing security.

I'm not saying everyone has to jump with every release of MySQL, PHP or Zen Cart. However, in the world of the internet and e-commerce, just two years behind can make a huge difference in the operation of your store.

One case in point was an upgrade to 1.5.7b from 1.5.3. The store owner was concerned that business was getting slower and slower. Had no idea that there were nearly 100,000 debug files in the logs folder. In 1.5.7#, it's just a simple click to have Zen Cart let you know when something goes bump in the night.

Bottom line here is that I fear you are going to try to run two stores with neither having a solid foundation. Please find a way to get to 1.5.7b with PHP 7.4 and make your foundation solid before putting two stores on it. Leave the old store running and build with the new. Don't make matters worse trying to continually put bandaids on your site.

This mod can be very helpful and, with DrByte's efforts, you'll more than likely have a working solution IF you get the foundation repairs completed.