I went back through all the posts thus far in this thread to prepare the summary that is below. If I've missed something, by all means post a reply. Keep in mind that laws on taxes, inheritance, etc. vary greatly from country to country so, no matter what, consultation with an appropriate attorney will be necessary in any plan.
Although I've tried to categorize things, some items don't fit neatly into any scheme. However, my hope is having an organizational scheme will make it easier for people to use as well as prompt additional suggestions, realizing that no one size fits all owners and all countries.
Consult attorney and/or tax advisor on the following questions:
-- given your own circumstances should you set up a family trust, or is it sufficient just to make sure the vehicle titles also are in your spouse's name?
-- should you gift the motorcycles now to sons/daughters? There may be a time rule (e.g. 7 years) to avoid inheritance taxes.
Preparing for the Inevitable: (in all cases make sure your spouse knows where the relevant documents are located)
-- get clean titles to all motorcycles and basket cases that are in your garage.
-- if necessary, change the titles of all the motor vehicles to be in both yours and your spouse's name, or in the name of your trust (the choice of wording on motor vehicle records is important; "and" doesn't mean the same thing as "or").
-- compile a book or spreadsheet with details and notes on important numbers on everything in the garage (e.g. +1900 for a Vincent), and with your estimates of the current worth of each item.
-- get important documentation from owners clubs on things that affect the value (e.g. a letter from the BSA OC showing your Gold Star's engine and frame left the factory together).
-- gather into one location other documentation that might affect value (e.g. your bike on the cover of 'Classic Bike', a letter from Steve McQueen saying he won the ISDT on it, etc.) .
-- arrange a mutual assistance pact with trusted friends to help your widow.
-- make list of friends who can be trusted, and those who can't be.
-- investigate the "best" auction house and venue for selling bikes and parts like you have (e.g. Sothebys' January Las Vegas auction). That is, if you were going to sell your bikes yourself today via auction, which would you use?
Possibilities for selling complete bikes: (figure out which is best for your collection and leave those notes for your spouse)
-- eBay listing. This may be a good choice for popular machines.
-- for people in the UK, have the VOC Spares Company sell it.
-- consign to auction house (will only net ~75% of retail after paying transportation and commission).
-- a reduced commission might be negotiable.
-- consider offering to auction house as 'no reserve' to help negotiate lower commission.
-- sell as a lot to a dealer (investigate the names of appropriate dealer(s) and leave those notes for your spouse).
-- turn over to someone you recommend to sell bikes for you, based on either a fixed commission (e.g. 10%), or on a fixed value (e.g. your spouse wants $20k for a given bike and anything over that is theirs).
-- gift to a non-profit museum (consult tax advisor to see if this helps financially).
-- advertise in club magazines.
Possibilities for selling parts: (this is more problematic because of the volume of parts and probably their lack of organization, the difficulty of identifying each part, listing, packing, and shipping it.
-- turn over to auction house to sell. Likely will realize only 25-50% of retail value, and not all auction houses even will deal with parts.
-- turn over to someone you recommend to sell the parts for you, based either on a fixed commission for each part or on a fixed value for the entire collection.
As a final note that applies to me as much as to anyone else, don't let the impossibility of preparing a "perfect" plan detour you from doing as much as possible. Difficult as everything above is to do, it would be much more difficult for anyone other than you to do it after you're gone.