Depending on when you want to do this, of course you can. Pretty sure 2008 is full.
I've ridden the Engadine marathon (but just the 100k).
These are not really races... unless you make it one, but you have to make the time limits.
9:45 or a 14 kph averageStart: 6:45, in blocks
Time limit
Corvara direction Campolongo (55km) 11.15 a.m.
Cernadoi direction Passo Giau (76km) 12.00 a.m.
Finish: 55km 2 p.m. 106km and 138km 4.30 p.m.
If not, you will be re-routed onto the shorter courses.
Perhaps start with something smaller like the Engadine one, or Stelviobike - this is a lot of fun.
Just ride and climb lots. To know you are ready, you have to be sure you can do the 10 climbs totalling 4000 meters at the minimum speed to make the cutoffs. Perhaps 2000 km would be good to have trained.
p.s. NO, you are not crazy. for crazy, try the Alpenbrevet. 260 k, 6000 meters climb. I just see they changed the route - make that 7000.