You can use a simple triac-based dimmer to control speed of this motor. Throw away everything apart from motor and C1 (which is very likely to be mounted directly on it).
Such dimmers are used in filament lamps dimming and in mains-powered drills
Google "triac dimmer".
Some implications:
- it might be tricky to fit a dimmer into the existing case (of cause, you can put a dimmer outside the mixer, but it will be weird

- don't fiddle with mains if you don't have enough experience. It can be very dangerous.
I do appreciate the love to some old ex-Soviet appliances, but can't understand this :-/ (the quality is usually very poor)
Why not to buy a normal mixer with proper speed control ?