Well, ain't that a kick in the head!
Take a very close look at your prop shaft, and I believe you'll note that there is a "shoulder" on the shaft, close to the oil seals. The propeller thrust hub sits against the square-shaped shoulder. Unfortunately, the old-design thrust hub is no longer available.
Since you lost all your stuff, there is a way to upgrade to a newer-style prop. First you need a spacer which will sit against the prop shaft square shoulder. The P/N of this spacer is 23-86918T and is still available from Mercury.
There is also a very thin, split washer that goes between the spacer and the shoulder of the prop shaft, P/N 12-31013 which is still available. I've seen many motors run without this thin spacer, it always seems to get lost. But since you'll be ordering parts, might as well get it!
Next, you need the newer-style thrust hub, which is thinner than the old one. The 23-86918 spacer makes up the difference. P/N for the thrust hub is 13191A 1.
You'll need a propeller nut locking tab, still available, P/N 14-816629Q.
Last but not least (besides a prop), you'll need a prop nut. This one is rather unique, 'cause the newer motors' prop nuts have fine threads, whereas the older motors have a coarse-threaded nut. The difference is very apparent. So if you were to go to a Mercury dealer, they might not stock the older coarse-threaded part.
Here's the P/N specified for your motor: 11-29594
Check out this parts diagram and you'll see how all the parts "stack up":
Mercury Marine Model 850 85 HP(6 CYL.) GEAR HOUSING ASSEMBLY, COMPLETE (MERC 850E) (PAGE 2) Parts
Now, for the tricky part, the prop selection! Pretty dicey, not knowing what was on there before. Given the size of the boat, I'd start with a 17"-pitch 3-bladed aluminum prop (or pick up a 17" composite Comprop, they're cheaper). See how the boat runs, if you've got a good local dlr they'll let you return an undamaged prop if it's the wrong size, and swap out for one with a different pitch.
If the motor revs up too much with a 17"-pitched prop, you'll probably need an 18" or 19" pitch.
If the motor "bogs" on takeoff, the pitch is too steep and you'll probably need to go down to a 15" pitch.
15"-17"-19" are common sizes and all are within the realm of possibility for your boat/motor combination. I guarantee that the 15" will "RIP", but might be too small, hence try the 17" first.
The prop you'll be looking for will have a more-or-less flat surface on its forward part, where it bears against the thrust washer.
Newer-style props don't require a separate thrust hub at the rear of the prop, since the thrust hub is built-in. So when you look at that parts diagram, don't be confused 'cause there are more parts than you expect. You're better off upgrading to a newer-style prop;since you have to buy one anyway you might as well take advantage of the modern designs.
When installing the prop, spread waterproof boat bearing grease on the prop shaft and threads. Tighten the prop nut with locking tab, using a block of wood between prop blade and underside of the lower unit to keep the prop from turning. Use a firm amount of torque but don't overtighten. Tap down only one locking tab until you find out if it's the correct prop or not. Once you know the prop is good, tap down all 3 tabs with a screwdriver/hammer. It won't fall off again!
HTH, see the pics below of the thrust washer and spacer.........ed
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