I'm assuming they are 8ohm, as they are being run from a Macintosh amp somewhere, and being run in parallel with the tweeter, which is probably also 8ohm, letting the amp see a 4ohm load over the 4 speakers.
Replacing it with another 8ohm speaker, is definitely the best option in that respect, if you're not wanting to bypass the amp.
What speakers you can find in an 8ohm configuration will just come down to a google search, I don't know of any off hand
So that one from Jaycar would definitely be an adequate replacement. If you're replacing one, then I'd recommend replacing the other side as well. if one has bust, chances are the other isn't far off busting either.