Picture time
I solved the choke problem today. I went "hard way". I drilled two spots on the choke shaft through the hole (do0n't let any chips to get into carb): Choke fully open and fully closed. Than I made a spring plate 0.35mm - thickness gage is made of very good steel

. I drilled two holes: 6.25mm (for carb screw) and 3mm (for ball) 6.3mm apart. For locking I used 3.25mm steel ball (from some old ball bearing). It works very well. Even with the spring hooked the choke can be lock in the closed position.
It was not difficult at all.
So here are the pictures of my new Choke-lock.
RysiuM