ok, I've done a little research ...
getting from Zagreb to Krško i think is no big problem, the problem is getting back.
you only have one train, and it leaves the Krško station at 22:34, and you change at Dobova, where is a 15 minutes break.
as Rob already said, parking the car at the stadium is no big problem, but don't arrive 10 minutes before the GP
Mr Snackette, the distance from Zagreb to Krško is not 60, but it is not 30 as well
Krško stadion has a nice web site, also in English, you can check out all the details there ...
speedwaykrsko.si