Connecting an old hard drive to recover data.

Unfortunately, your old drive (Apple Part #655-0141) is a Quantum Fireball 1 GB SCSI hard drive with a 50-pin interface, dating back to the mid-'90s. There is no feasible way to connect it to your iMac, because it would require more than a physical adapter to convert it to one of the iMac's USB ports. If you had a Mac with PCI slots, adding a SCSI controller card would be an option. You might want to check for any Mac User Groups in your area, to see if you could take it to somebody and have the files retrieved from it. Buying a compatible, older Mac to connect it to would be the last method to consider in terms of expense. A 1999 B&W G3 tower with an Adaptec 2906 PCI card would enable you to connect the hard drive, and then you could transfer the files to a USB flash drive. This would require having a display that's compatible with the B&W's VGA display port. Acquiring an assortment of outdated hardware for a one-time retrieval of files isn't a realistic solution.

I suspect it is from my old G3 or eMac...

I'm thinking it's older. Most Beige G3s had ATA drives of min 4GB capacity, except for the top-end model that had a 9.1GB UltraSCSI drive powered by a factory-installed PCI adapter card. (I have one). B&W G3s were all ATA/IDE.

The first PowerMacs to have a SCSI interface and the option for a 1GB capacity were the 8100s.

All eMacs had ATA/IDE drive interfaces and the smallest eMac drive offered was 40GB.