Good question at first. Although I doubt she will user her powers on kuma.
BUT if she dies!?-- Unless she tries to turn him into a child he should be fine. His body will be the same size so his cyborg parts wont fall out.
And surely bonney's powers reverse age but not damage(kuma's cyborg surgery). could you imagine how OP bonney's powers would be if that is how they worked?
"Oh no ace got a hole punched through him by akainu, quickly bonney! Revert ace to an age where he didnt have a hole in his chest"
So realistically, nothing should happen to kuma. Unless bonney reverts kuma to an age where his body is very small. Then the cyborg implants might fall out, or just look really stupid in him (like franky's look now) but regardless of what bonney does kuma will never get his mind back. Not from her powers.
She's personally connected to Kuma, and she hates the current mindless slave-state he's in. Using her abilities to fix that seems like a good call imo.
We don't actually know that second thing, and it's an interesting question. Would his chest just explode because his organs don't fit with the machinery? Who knows!
And you're right, it would be OP for sure if the potential was fully used, but then again, so is a lot of stuff and we won't know until we see more from her.