Finding a floor tom to match wont be easy, but I think there are some options there. The bass drum is a 20 inch, correct? Is the rack tom a 12 or 13"? If it is a 12", I would try to pair it with a 14" floor tom. If the rack is a 13", I'd pair it with a 16".
16" MIJ floor toms come up somewhat regularly on ebay, and if you found one that was Star made (with the same style lugs) it would be a match. Also, a 16" floor tom of some other type can easily be found on ebay or CL and recovered. I just checked the wrap section of George's Drum Shop site and the blue diamond pearl actually looks pretty close. So a rewrap could be done on almost any 16" floor tom.
If the rack tom is a 12" and you want to go for a 14" floor tom, aside from being very, very patient, I would consider purchasing a keller shell and building the drum from scratch. I dont think this would be any kind of a sin since this set originally came as a 3 piece. The reproduction lugs wouldn't be exact, but they'd be pretty close. Also, you could even find the Star lugs on ebay and drill a shell to match those (I dont really recommend doing this as I think that, along with the tom mounts, the weak links of these drums are the lugs. They are cool looking, but are made of pot metal and aren't nearly as durable as new lugs, but it is an option.)
I dont know... it's just a cool set and it is going to be worth your while to get it up and running.