Apologies, I hadn't seen the previous replies (IO's and yours).
The route you propose definitely takes two days. A fuel and overnight stop in France would be required (somewhere like Chambéry, for example, so you have the range to get to Slovenia), then an early start the next day should see you in Romania in the afternoon. Overnighting in Croatia would be another option, but that relies on your making an early enough start from England so you cross the Alps in broad daylight--even so, it's preferable to cross in the morning due to atmospheric stability. It can get annoyingly rough latter in the day.
Can the aeroplane do IFR? If so, taking an IFR-rated pilot with you might be your best if not only bet.