When rebuilding my 87 Truckster I found that a local forklift repair shop had access to some kind of cross reference list and could get me many of the steering and drive train parts that I needed. If I had the Cushman part number it helped them, but they could find parts without the number.
The local ORiley Auto Parts had an older guy working there that had been in the auto mechanics and parts business for years and he was also very helpful. If I brought in a part, he could look at it and remember if it was a common auto part, then go to the shelves and open a few boxes, and usually come back with an identical replacement. There's no cross reference listing for Cushman available at the auto parts stores. It was in his head from years of experience. The parts guy at the forklift place looked at my master cylinder, went in the back, and came back with a new identical master cylinder. The price that I paid for it was 1/2 what they are going for on Ebay. Brake shoes and wheel cylinders were also available from him for great prices. His comment was that they were also used on some older fork lifts, so he kept them in stock. Fork lift repair shops also repair all kinds of industrial vehicles, and also kept the Cushman Trucksters repaired for the local factories. You won't find sheet metal body parts this way because they were manufactured by Cushman, but many of the mechanical parts used in these industrial vehicles were standard OEM automotive or fork lift parts. The trick is to find someone who recognizes it and can find it on the shelf without using the computer or printed cross reference. I had a local sheet metal fabricator build me some sheet metal that I needed. I gave them the rusted out parts and 3 days later I had new ones, minus the paint and the price (simple parts) was quite reasonable.
The 18 hp OMC motor is almost identical to the 22 hp motor, except for the cylinders and the advance springs in the timer. An 18 hp can be converted to 22 hp just by replacing these parts. The cylinder differences are easy to spot. The 18 hp cylinders have the spark plugs on the lower side of the cylinder while the 22 hp cylinder has the spark plug on the upper side of the cylinder. Both cylinders on the engine are also identical. There is no right or left when buying new, but keep them in their original positions when repairing the engine. I'm not certain if the 18 hp cylinders are still available new, but the 22 hp cylinders are still available new from several of the sources that you have listed. There are also complete 22 hp short block engines still available new. The 828767 manual Supplement contains a lot of information about the 22 hp engines so most of it also applies to the 18 hp engine.
Go to the Jacobsen website www.jacobsen.com and search their manuals. They once owned Cushman and have been very helpful. If you don't find a listing for your Truckster, call them. They still have many manuals that have not been scanned and converted to pdf files yet. A request for something might at least get you a photo copy.
If you try these places and still can't find what you are looking for post a request here. Pictures can be very helpful if you an post them. Several of us should be able to help you, or at least 'Unforgiven' and me 'CharleyL'.
Charley