I'm a big fan of Marzocchi forks, too. I've got an '01 Z2 Atom Race that's just been bombproof and buttery smooth. The forks are easy to service for routine maintenance, but I'm sorry, I really have no idea what you'd be getting into in replacing the lowers, cost or otherwise. On your '02 fork, I'd guess the stanchions are 30mm diameter, FWIW. Also, the stock oil in your fork was probably Golden Spectro brand motorcycle fork oil, in 7.5wt. Any synthetic motorcycle fork oil ranging from 5wt to 15wt., depending on preference, should work. I will say that with the fork being cracked, I wouldn't ride it anymore. Cambria Bicycle Outfitters has carried a fairly good selection of suspension fork replacement parts in the past, you might look at their website or better yet, give them a call and give them the info on your specific fork and see if they can help you out.
www.cambriabike.com