The speed of the motors will be determined by the frequency and load rather than the voltage. Luckily, the frequency is usually rather stable, but in a poor country like Haiti I would expect it to vary up to +/-5%, maybe more.
An unstable voltage indicates some additional problems: The motor is only designed to handle a certain undervoltage (10% or similar). Even if you boost the voltage 20% to accomodate the higher frequency, the motor could still be damaged if the supply voltage falls. The impedance of the distribution network is often high in poor countries, meaning that a large load (like the motors) will cause the voltage to drop considerably below what you measured.
To make matters worse, if the phase-to-phase voltages aren't within 2% of each other the motor will run hotter than normal.
All of these potential problems can be addressed with transformers, variable frequency drives and other tricks. But before you run out and buy these things: Check how much they cost. There might be a simpler solution: Putting in one size larger motors. A 4 KVA motor is not very expensive.
The cheapest solution is to see if the load on the conveyer belts can be reduced and run the existing motors off the existing 220V.
