We've got them, so we need to mechanics them.
Okay, initial proposition - 4 basic grades - gawd-awful, basic, modern, Military
God awful is "you have a hook or a pegleg!" They suck. These are mostly seen in emergencies where doing a proper job is impossible due to some external fact - like you are in the middle of a war zone. Characters recover 1 point of either speed or finess. when used in a strenous action, make a Health roll against the target number of the stresed action - failure implies you overstrained you damaged limb against the prosthetic and you now have a basic Bleed condition (1 wound, 1 round, 3 duration, medicine check 7) where the two meet. This level of prosthetic is useless for anything other than a limb, or as a temporary dermal patch (make a check each hour (generally, Hardiness+d6 vs (20-Medical check result to apply +1 for each previous check made) - failure implies an infection has set in (1 wound, 4 rounds, until cleaned and treated, Medicine check 10). All touch checks made through these sorts of prosthetics are at -6.
Basic is only slightly better than what we have now - it's a step up from a peg leg, but not particularly good. It restores any stat decreased by limb loss to 5, or the intial stat-1, whichever is lower. Agility also takes a -1, just because the expected balance is off. The replacement is obvious, being the bare minimum needed to be structurally sound - in many places, the use of a basic prosthetic implies poverty, and will result in a minus to social roles when dealing with people who care about such matters. While they wouldn't look out of place here and now, the micro-motors and servo controls make them significanty better. Unlike godawful, it is possible to "brace" additional support structures, and producing benefits to those who have lost certain other pieces - a poor replacement ear, for example. Using this grade of prosthetic to detect something is done at -4, or 1/2 the deficency in the case of replacing a single part of a paired sensory suite (1 of 2 eyes or ears) This grade can not be used to restore lost organs, though one may serve as a temporary stop gap for Health*40 beats before the Dying Affliction sets in. Certain forms of basic prosthetics may be made to enhance thier functionality, but are nearly useless for sensing, and rarely work any better than a god-awful one outside their designed pupose. Legs can be modded to enhance Speed or Agility, and arms may be modded to enhance either Force or Finess, for a single type of skill or action ("blade" style legs could have a 7 speed for running, but would be considered god-awful for jumping or climbing, for example). This modification can be no greater than the original stat+1, and may not go above 7. These modifications are blatantly obvious, and their function is readily apparent. Replaced sense organs may not be similarly modified in this fashion. Basic Prosthetics are rarely damage resistant, and will break if attacked, or used outside of "normal" operations.
Modern Prosthetics are what most people are wandering around with. These are made to mimic the lost structure, and can handle replacing any body part except the brain and spine. Minor replacements result in no penalty at all, while major ones generally lower Speed (and Agility if the replacement is unbalanced) by one versus the pre-amputation stat (just because the replacement is so heavy compared to what was replaced), but is otherwise just as good as the real thing. Further, they are resilient enough that
[to be finished later]