So...my real feelings on what you propose with your saddle...
Your saddle is
a family heirloom.
The horses do not
belong to you, you will just be allowed to ride them if, if your saddle can be used.
Your friends want to customize that heirloom to fit one of these horses at I bet a pretty good price...sounds like you are expected to pay for.
You are referring to a large amount of money to spend to correct some issues the saddle has do to age and the leather is old and been sitting, it has not been in continuous use... So, no to using that heirloom on a horse that is not yours.
No to investing all that $$$ cause it is going to be considerable.
I would instead purchase a used saddle in your size
that is comfortable for you, in good condition and will fit the horses well with maybe a modest saddle pad needed and leave the family heirloom alone.
If this was "your" horse it would be different, but you don't own and you never know what tomorrow could bring in friendship. For that reason I would not
allow the use of that special family heirloom irreplaceable.
When you purchase this saddle it remains your property and make sure it is marked permanently with your name/initials so you can identify it...again, friendships can & do change.
As for stirrups...
Lately I've seen pictures of and read articles on those composite things breaking and the rider being injured, sometimes badly.
If you feel you need to ride with peacock stirrups then buy them and a extra set of rubber-bands.
If you just want a "English" stirrup them buy fillis pad stirrup with a white pad.
If you want a stirrup that has a wider footbed for more comfort if you trailride for hours over varied terrain, then look at stirrups that endurance riders use or trail riders use.
I would not
mix true western stirrups though on the English saddle as the safety bars may not release you properly if you fell and got hung up.
I'm also not into spending oodles of money when the standard riding saddle accompaniments work well and are safe to use.
If you bought a used saddle, often it comes with leathers and stirrups on it..
If you bought a new saddle then those items are extra but you can buy affordable not need to spend top-dollar unneeded spent.
I thought you had had the saddle evaluated already for bringing back into safe daily use condition and it was very costly....
Remember too it probably will not
be just you using said saddle, and although you may take really good care of others tack...not everyone reciprocates that way and this is your heirloom you refer to being put out there.
Honestly, I would look for a used saddle local or a entry level new...
For what I imagine a leatherworker will charge for the work needing done, plus someone to re-flock for saddle fit you could buy a Wintec with adjustable gullet that would fit many a horse. https://www.statelinetack.com/item/w...ocked/E006280/
There are so many variations, choices and prices for any of these I quickly gave links to...
Other places I would do some searching if wanting "new" is www.doversaddlery.com , www.chicksaddlery.com , www.jeffersequine.com
and then of course are the hundreds of places you can find with a computer search.
The places I listed are all trusted tack shops with good reputations and customer service to assist you.
Look local in trade-papers, local tack shop, feed stores and put up flyers of what you are seeking and see what you find if you will buy used .
Facebook has thousands of tack groups all over the world...
There are local/regional groups everywhere.
I just would not
do what you comment about with a treasured heirloom on a animal that is not mine and I will not
have total control over who is using it, just would not happen to my treasured saddle.