I assume this is the large pad of acetyl that is fixed to the keel. It's hard to envisage how it has come away without a photo to look at.
I would think that the best thing is to replace the damaged pad. If faced with that job, I would probably get a suitable sized block of acetyl from Direct Plastics or some comparable supplier. It would than be a job of shaping the pad and drilling the holes and counter-bores. I think most of the pads have some shims [usually penny washers] behind them to get the final space out right to match the keel box.
I've had to mess about with most of the other bits of the keel on Dark Star, but the pads on the keel itself haven't been a problem as yet.
I guess you have been able to take a look at the bits at the bottom of the keel box. While she is out of the water it is quite easy to have a look at the forward roller and the aft buffer - both of these are bolted in to the bottom of the keel box area.
Congratulations on acquiring a 275! I have just got mine back in the water, and am looking forward to some sailing now.
Regards