I used the excellent FEIN Multimaster w/ their black saw blades to cut the cabin sole out on a 45 degree bevel. When I replaced the rotten floors and a soft hull section, I used thickened West System epoxy and bedded the original sole back in. You might make a cardboard template of yours before cutting it out to aid in fitting its replacement.
If I was faced with your problem I'd explore the condition of the sole around its perimeter with hopes of the same strategy. If the perimeter is serviceable, I would just replace the defective area. It took us about 6 work hours to cut out the sole.
If you have a problem buying marine plywood in your area, see if there is a highway sign company near you. Around here they sometimes use marine plywood and seconds may be available for little or no cost.
If you decide there is no way to save the edges and insist the sole has to be one piece, then you will have to remove almost the entire interior. Frankly, I think that is un-necessary as long as you tie your replacement parts back into the serviceable remains
of the original sole. The salon faces just stand on the outer edge and you should be able to epoxy on to that almost as strong as the original (now old) plywood...
These are not problems designed in by Bob Perry. His design was purchased by Islander Yachts and their people did the engineering we bought.
I28 Milwaukee, WI
Not a marine engineer - just what I did to repair our boat...