Most of these 912S powered machines run at fairly low power settings - so less than 15lt/hr and probably 70kts ish. With that additional tankage that amounts to up to 10 hours and 700 miles - if the tanks were full.
But with a hop across the Channel they may not have been.
But with those sort of modifications the machine would have been able to fly long legs - pretty necessary for the later parts of the trip.
Stall speed is given as 40mph and the runway requirements about 200m so much more like light aircraft requirements than earlier weightshifts.