This is a common question, so I thought I'd put down some views.
New Frame Option
The first thing to say is that we don't make frames, but we do have a local partner that does.
Frame builders often use a single extruded section, possibly in one and two channel variants. Larger builders (eg Seaglaze, Trend etc) will have several sections available, but even still, it's going to be a short list.
When I first started looking for a section for a Client with a Moody Eclipse 33, I naively thought it would be a case of selecting a section from an extruders catalogue and then getting a builder to use it to make frames, rather like selecting some paint for a decorator to apply. But no, this is not how it works.
So if your frame builder happens to use a suitable section, then you've lucked out, but if not your only option may be to trawl around others to see if they have something better. Not a great start!
Even if you find a section, these are now bespoke frames so will come with a bespoke price and will probably take 6-8+ weeks to create.
One last thought (related to the Eclipse). If your 'new' frame section differs in size, weight or stiffness to the old, there can be issues refitting, particularly if the hull/coachroof section is not completely flat.
New Frame Consequences
A new frame sill stand out. If mixed with old this will be in colour as well as probably section. New frames look great off the boat, but when fitted will likely make the rest of your deck hardware look 'more' tired, so unless your refit extends to all hardware the new frames probably won't enhance the look. Of course this is made worse if only replacing some frames.
Old Frames - the case to save
Most frames are pretty robust, even after 40+ years. They will have patina in the form of scratches, discolouration and inevitably some corrosion (particularly around/behind screw holes and section joins). Older frames will likely be fixed with chromed brass fasteners, whilst later units will have stainless steel (which will cause slightly more galvanic corrosion).
But, old as these frames are their condition and look will be consistent with other deck hardware, having all aged together.
If leaking then it's likely there will be signs of corrosion around the channels holding the panes. This will clean up during refurbishment and when resealed with silicone is less likely to reoccur. Similarly corrosion around/behind fixing holes will largely clean off and when correctly rebedded be less likely to worsen.
In the limit we could re-anodise but this is rarely cost effective or worthwhile as anodising old metal never works as well.
If you have inner rings which have corroded we can have new anodised ones made.
The bottom line is that in most cases, you're better off keeping the old frames and once resealed and correctly rebedded these will give many more years of faithful service. They will also look the part amongst everything else tired onboard, including the owners!