Looking through the PDF "MiG-21 Pilots Flight Operating Instruction", point 23 on page 23 reads as follows:
From the Emergency Procedures part of the same PDF, under Approach and Landing with Dead (Windmilling) Engine, one paragraph reads:
- in gliding, avoid (if practicable) abrupt deflections of the control stick; in the case of insufficient windmilling RPM, hence an insufficient pressure in the hydraulic systems, disengage the aileron boosters;
Further reading through this section, it seems if the engine is windmilling, you will get some hydraulic pressure. But do keep an eye on the pressure. If it drops, abandon the aircraft. The electric booster pump seems to only be for the booster hydraulic circuit, which runs the stabilizer two-chamber booster and the aileron boosters?