So after a day of removing the existing floor and cutting back all the rotten wood it doesnt look as bad as it first seemed (still pretty grotty, but I think do-able)
There are/were 6 ribs, one had already been completely removed, 3 are fine and the 2 middle ones are rotten. They are in 3 sections and the middle section of each is gone, the outside sections are perfectly solid. So it looks like I could just replace the middle bits but Im going to do that AND add a complete rib alongside the existing one bolting it into the solid bits. I will then glass it all together and it should be twice as solid as the original.
The existing outboard engine fixings were rusted solid so after much pulling, pushing, prizing, hammering and every other type of effort we ended up cutting a bit of the transom top away to get the engine off. This revealed a very solid transom top PHEW!!.
I then checked the existing engine well, which was full of expanding foam, sealant, rubber and just about everything else that could trap water and found it was soaking wet with lots of rot so that too was cut away. This revealed the inside of the transom so I took the opportunity to cut some of the internal glass away to check the integrity of the transom wood and once again it looked good
So after a day of cutting out rotten wood i think i'm happy