1) Start growing the fan base of this organ at least 3 years before the project starts.
2) Involve the clergy as much as possible.
3) Involve the congregation as much as possible.
4) Avoid the temptation to find the cheapest solution for restoration.
5) Before you start anything, check with the agency in your country which protects the cultural heritage about the proceedures in dealing and restoring with historic monuments. Sometimes you can find partial funding through them, too.
6) Gather a group of expert advisors who have necessary qualifications to do consulting on organ restoration.
7) If there are unfriendly people to organ music in your congregation, make them a part of the committee - it's better to solve the differences this way. Very likely if you give them proper attention they will convert to fans of the organ later on.
8) Look for donors for your project among congregation members.
9) Local businesses can become great supporters of your organ.
10) Look for businessmen among parents of the members of the youth choir.
11) Municipal funding is also fine when you have a partial support in place already.
12) Educate people of the congregation about the organ and its music by writing short articles published in church bulletin every Sunday.
13) At the end of every Sunday service go to the place where announcements are told and briefly talk about your project, how much funding is needed, how much you have so far, and how much you still need to raise.
14) Don't forget to thank regularly everyone who donated.
15) Take advantage of Kickstarter (only for projects in the US or UK currently).
16) Play yourself in fundraising recitals regularly.
17) Involve the members of your choir in fundraising events.
18) Have a thermometer (in the shape of the organ pipe) showing your financial
situation in the visible area of the church.
19) Put posters about your project on every bulletin board of the church.
20) Initiate "Adopt a Pipe" program in your congregation.
21) Display an exhibition of high quality pictures of the organ and its parts with vivid descriptions in the area around pews.
22) Produce brochures and booklets about this organ and the project to pass around the congregation members.
23) Start a blog with a newsletter in which you regularly document the progress of the project (you will find donors this way, too).
24) Start a page on social media sites about your project in which you can share articles from your blog, pictures, videos etc.
25) Choose a respectful organbuilder who has necessary qualifications and previous experience in restoring the type of organ that is in your church.
26) Oversee the work of the organbuilder so that he or she is very careful in dealing with historical material.
27) Many organbuilders will want to replace the old parts and pipes with the new ones, re-paint the organ and so on. Resist this approach as much as possible. Make them treasure the original material with utmost care.
28) Include a documentation phase in the project in which the organ and its every part is carefully measured, photographed, drawings would be made so that it could be recreated in case of fire or some other calamity.
29) Ask the existing friends of your organ to help spread the word about it online and off-line. Figure out what and how you want them to spread the news and make them easy to do so (for example, with one click of the mouse).
Although this list is incomplete and can be continued, I hope you will find more than enough ideas to start with.
By the way, do you want to learn my special powerful techniques which help me to master any piece of organ music up to 10 times faster? If so, download my video Organ Practice Guide.