The guy wires are structural element for stabilising the incinerator. People put them in if the superstructure proves too flexible in the wind as most incinerators should have been designed as free standing structures. Complete or partial removal of the guy wires can lead to damage of the structure, especially if the incinerator can be excited by arcoss wind oscillations (vortex shedding).
There is a very good chapter on guyed stacks in DIN 4133.
Do not remove any of guys in the incinerator for any extended period. When the incinerator shaks the base can fail by fatigue. You also need a qualified structural engineer, unless you are one yourself, to mess around with the base plate. In the absence of calculation I suggest do not use plate thinner than the existing design in any replacement. It is likely that you will need to engineer some stabilising measures while carrying out remedial work on the base plate.