[bojan]

This is how $2 Digispark ATTINY85 board fits onto existing controller PCB without any modification. I may solder the chip directly on board later, but it's mounted perfectly OK as it is now.
I also reverse-engineered the controller circuit to some extent..
HT66F018 (faulty now) is full-blown 8-bit processor, but it's only function here is generation of PWM from pot position (exactly what Attiny85 with my program from previous post does).. and nothing else.

Overload protection is facilitated by separate Opamp and comparator IC's, signal from this circuit disables PWM externally in separate wired-OR circuit.. Maybe they used here processor from, say, vacuum cleaner power regulator to save the effort of designing more sophisticated firmware? This would explain my troubles with minimum head speed....

Anyway, it seems I will have exactly the same functionality after this modification is finished, except I will have the desired lower speed.
And another controller board as spare :-)