The description of the Petrify spell (Shadow Spells, page 22) says:
Type: P Range: LOS Duration: S Drain: F – 2
This spell transforms living tissue into calcium carbonate, or effectively, stone. The spellcaster must succeed in a Spellcasting + Magic [Mental] Opposed Test against the subject’s Body (+ Counterspelling). The spell’s Force must also equal or exceed the subject’s body. The subject is not conscious while under the spell’s effect, and any damage suffered by the stone-like subject translates normally to the appropriate Condition Monitor. While in this form, the subject has a Barrier Armor Rating (p. 197, SR5) equal to their Body + the caster’s net hits when they cast the spell. Non-living material is not affected by the spell, including clothing, cyberware, and carried gear.
For the Petrify spell, you only take into account the base BODY of the target, and ignore any kind or armor or protection the target may have since the spell targets the flesh only.
Following the basic rules, from the core rulebook, a full cyborg character a la Robocop, despite being not much more than a brain and spine, still has his full base BODY to resist the spell, since the cyber only makes one lose Essence.
Is there a rule that would allow to recalculate the BODY of a character for the purpose of the spell, taking into account the cyberimplants?