James I am hearing warning bells here.
For the sake of further discussion, just scanning quickly through the previous threads and photographs: The hull has been heavily built in a work boat tradition, of carvel planking (yakal below and teak above the waterline) over sawn frames of white oak.
At a later stage in its life, the seams were splined and the hull covered with a heavy fibreglass sheath.
About a year ago a deck leak was discovered to have caused rot to one or more of the sawn frames, which the previous owner claimed to have "sealed and fixed in 2017 using epoxy". Evidently further water damage to frames has now been discovered, and some rough sawn white oak boards and epoxy was to be left to the new owner for further repairs. (Ref the original "for sale" advert.)
Photographs show at least one of the sawn frames has rotted away in places. Photographs one, two, three.
James, I presume these frames are meant to be part of the structure. In that case the rotten parts need to be removed, the frame(s) restored and properly re-fastened to the planking. That is, provided the planking itself has not been infected by rot.
Modern resins are wonderful stuff, ideal for modern methods of construction in association with modern materials. They have their place also in some situations in the repair of traditional hulls, but not in all situations. The idea of making "fibreglass ribs" in this situation does not seem to me like a sound idea, particularly if the reason is an intention to just try to stick them on from the inside.
The amount of timber required to make or repair a sawn frame is not great, so cost should not be an issue here. Shaping and fitting a sawn frame (or part of a sawn frame) to the original workboat finish might require a little patience with scribing, and a little skill with sharp tools, but not too much of a challenge if you study the existing frames to see how they were made and the hull fastened to them. I should think any durable timber would do, and I would be interested to know from an expert if there is any good reason why you can not fasten a new frame in place, properly, from the outside, and then repair any resulting minor damage to the fibreglass sheath quite simply with epoxy resin.
The above is not offered as advice, not knowing enough about the situation, but no rocket science is required here. It would seem not to be a major problem, and if a frame is actually needed here, as part of the structure, then it should be fixed or replaced properly. The actual advice that I will give you is that if you are uncertain about how to proceed, then it would be worth consulting a local boat builder, to come and have a look at the situation. Preferably not someone whose background is in modern materials but someone who has a knowledge of traditional shipwright skills. I am sure you can do the job yourself, but the hire of a shipwright or traditional boatbuilder/surveyor for a couple of hours, just to give you the right advice and set you in the right direction, might be money well spent.
(It might pay, at the same time, to get this person also to examine closely the condition of the planking, between those frames, as shown in photographs one, two and three above, and explain that matrix of ss fastenings. Hopefully the planking there is better than it looks in the photographs.)
(Referring to an earlier post, I would also be a little hesitant, and think carefully before following through with the idea of reinstalling the ballast with resin, as I suspect your idea of filling up to the level of the cabin sole might require a much greater volume than you may think. The use of resin here may be a good suggestion, but in your application of it there may be the potential for some further brainstorming. Anyway, that is another issue.)