Thank you both for the information. I called James Black some time ago when I saw his advertisement in Pilot magazine, but he said that the Zlin was off-line. I should try again. And I will certainly look into Carcasonne.
The 726 that I flew in Hungary had a 180hp engine. (One of the facts I had to learn before being allowed to fly - in Hungary you get examined by an engineer before being allowed near the controls. I'm not sure if I got all the answers right, or if my instructor friend covered up for me when translating! ) The engine can be supercharged as in the Zlin 142, which takes the power to 210hp.
The performance with 180hp wasn't spectacular, but a vertical roll was possible with (just) enough speed left for a stall turn at the top provided you dived to 280km/h and pulled hard. The handling of the 726 was delightful, though I have never flown another plane which made me use the rudder as much. With every change of speed or change of pitch, you could see the nose move to one side or the other. And what a view over the nose! The strobe effect of the propeller blades and the constant speed unit in aerobatics was mesmerising!
The Hungarian approach to aerobatics was fairly brutal (don't pull 4G if you can pull 6 instead!), and the result seemed to be a lot of rather tired Zlin 526s which couldn't be used for aeros any more. Instead they use them as glider tugs - fun for the pilots.
DB6 - what was the story with not being able to get your 526 flying? Do you still have it?