will my old track pad work with my new iMac?

The AA-battery-powered Magic Trackpad should work.

I’ll presume you’ve tried new AA batteries?

Catalina doesn’t differentiate supported trackpads: macOS Catalina - Technical Specifications

Reboot and try again, and reset NVRAM and SMC and try again, too.

How-to:

How to reset the SMC of your Mac - Apple Support

Reset NVRAM or PRAM on your Mac - Apple Support

Scrounged a Magic Trackpad 1gen and paired it with somebody’s iMac 2017 running Catalina, which is the closest combo I can source right now. Worked.

In theory it should work.

I was able to pair my 1st gen Magic Trackpad (purchased 2010) to a 2017 two USB-C port 13" Macbook Pro.

However, I was not able to pair to someone else's 2015 13" Macbook Pro (last model to have all the traditional ports; hdmi, USB etc...). Because it was someone else's Mac I did not try very hard to make it work, but since I was able to get it working on a 2017 model, I have hope it was just that I did not take enough time with the other guy's 2015 model.