Any alternative to replace variable frequency drive (VFD) to vary the AC motor speed?

Its mainly mechanical if not VFD.
Fluid coupling
Gas Turbine
Steam turbine

The speed of a motor is based on the formula N(rpm) = (120 x f)/p
where f = frequency of supply
p = number of motor poles.
therefore the only way to vary motor speed is to change the number of poles (2 speed motor) or change the supply frequency (VFD)
Alternately a variable ratio gearbox or Eddy current drive (obsolete) or some other mechanical means such as pulleys etc.

You cannot get a definitive answer to a vague question. There are too many varieties of "AC motor". Narrow the field by describing exactly what you have and options can be discussed.

Varying the voltage alone will only work on a very small segment of "AC motor" technologies by the way. For all of the others, it will cause them to overload and burn out.

Start with a mechanical system. A simple gear ratio an change to another base speed but does not give speed changing ability. A DC drive will do all that a VFD does but will have motor maintenance issues. An eddy current is a simple, brute force AC drive that gives limited speed range.

A 3 phase generator turned by diesel motor or DC motor with variable speed could be good.

How about an eddy current drive you still use an AC motor that connects to the Eddy Current Unit that will allow you to vary the final speed while the motor operates at full speed.

Although other methods exist to vary the output speed of a motor, the costs of the methods employed whether Electrical or mechanical in nature (including space, running costs, loses, inefficiencies, wear and replacement rates and maintenance) are often considerably higher than the investment in a stable voltage/current supply, good grounding, a VFD, and an energy efficient motor even when space or environment are issues. I guess this reminds me of the old adage "Pay me now or pay me much more later"

The speed of a standard Squirrel Cage AC Induction motor is determined by the following formula... RPM = 120 * Hz / #Poles:

Where 120 is a constant for three phase motors and the number of poles can only be even (N&S) and are normally fixed.

So the only way to adjust the speed is to change the number of poles or alter the frequency.

You can have multi-speed motors with multiple or reconnectable windings in order to change the pole#s. Other than that the only variable left to change is the Frequency.