If you have another PPC Mac that can run/install 10.3, there are a couple of ways you can attack this.
Probably the easiest way is to do use target disk mode on the iMac, although this is only possible if your iMac has Firewire ports. The early tray-loading iMacs do not, and since you're trying to install 10.3 I'm guessing this may be what you have.
In any case, first install the new hard drive in your iMac. Then, to install via Target Disk Mode(TDM), you first need to boot the iMac into TDM by holding down the "T" key on the keyboard while powering up. You should hear the chime, and eventually(a few seconds to a minute) a yellow Firewire logo will appear on the screen.
Once you've done this, connect your iMac to your other PPC computer via a Firewire cable. If the second computer is up and running, the iMac hard drive should appear as a mounted external volume on the desktop. This is also a good time to run disk utility and format the hard drive in the iMac. Select the iMac HDD from your second Mac and choose "Mac OS Extended(journaled)" as the format. Then, click on the "Partitions" tab and use the drop-down menu to set the number of partitions you want. Even if you are only going to have one partition, you still need to select "1 Partition." Then, click on the boot partition, hit the "options" button, and select "Apple Partition Map" in the pop up window that comes up. Click okay to everything, let Disk Utility do its work, and exit out of it. You can then run the 10.3 installer on the second Mac, and just choose to install on the Firewire volume-which is your iMac-during set-up(it should appear there).
If you don't have Firewire ports on your iMac, you can still use a second PPC Mac for installation. You can install 10.3 on the hard drive outside the iMac, and then transplant it into your iMac. If you have a G4(or B&W G3) tower, I'd suggest just putting the replacement drive in the tower and installing 10.3 in that way. If your other computer is a G5, an iMac, or a laptop of some flavor, you will need an external USB or Firewire enclosure for the hard drive that you then plug into the relevant ports on the system. USB enclosures-or even just loose cables-are all over Ebay and are cheap. Firewire ones are a little more expensive, but will make the whole process a lot quicker-especially if your other PPC mac only has USB 1.1 and not USB 2.0. Just be sure that anything you get will support a desktop(3.5") IDE(aka PATA) drive.
For any of the above, the procedure is much the same. Use disk utility to correctly set up the drive as above then run the 10.3 installer on the computer.
