We (Henry Willis & Sons) have done at least 20 Harmonium restorations/refurbishments over the past 20 years since I became Managing Director: these have included work on instruments by Mustel, Alexandre, Schiedmayer, Christophe et Etienne as well as the more common 'American Organs' such as Spencer, R&D Apollo, Estey and my very own Aeolian Grand Player Organ.
There is no simple way of doing a patch repair if it is intended for it to be able to be forgotten about for a lifetime - the internal leathering of a feeder rib joint for example is actually just as important, if not more so, than the outer bit which one sees. It might be more correct to refer Tony's comments to the matter of the leather to be used and not necessarily pipe organ builders - the techniques are identical, merely the materials which are different. Most current-day 'restorers' started off themselves as amateur enthusiasts, they are qualified by experience and not necessarily training.
If you have a year of your own time to spare and you don't mind being without the use of the instrument, you can have a go yourself and save the money, but if you want it turned around in a matter of a few days then pay an organbuilder or anyone else who can do this professionally the rate of about £25 per hour - much less than you will pay a washing machine serviceman and certainly a great deal less than servicing your car, even though it requires a vastly greater amount of skill.
By the way, don't be fooled by its looking like a reasonably straightforward job!