Well seeing as no one else has replied and a few people may like to know. I'll chime in.

The silver one that Sam's dad owns I think is a Ducati 750 SS Classic which i think is mid 70's (but I do think there was a 900 SS in mid to late 70's that was identical to the 750).

Now Sam's (bike ridden through the tunnel) one seems to be either a modified Sports Classic 1000 S with the bikini fairing removed (if that's possible), or a SportsClassic 1000 with a MonoPosto seat, either way the tail light is also modified to be an LED light by FLEDA I believe. The bike also has an exhaust on each side, which makes it either the modified 1000 S or a later model (06 to 08 maybe) 1000. The earliest models of the SportClassic 1000 had a twin pipe on the right hand side.

I stand to be corrected though as I am not an expert

750 SS Classic

SportClassic 1000

With Fleda tail light

As you can see in the last pic there is a twin pipe on the right hand side on the earlier models of SC 1000.

Hope this helps or can get a debate going should there be inaccuracies

 

Sams bike is the 2006-2008 ish Sport Clasic 1000 Model Biposto. Seat in picture below is the biposto.

Kevin Flynns Silver Ducati is the 900SS. This bike never appeared in original TRON 82 movie.
Robert Smith's bike was used according to this link.

The Last Ducati 900SS Was the Best - Motorcycle Classics

Of all the Ducati SS variants, the last-of-the-line 1982 Ducati 900SS model may have been the best.

www.motorcycleclassics.com

Looks like they dirtied up the bike seen in the garage of sam Flynns shack
Definately an 82 model looking at the tank and decals