My name is XXXXX XXXXX I will be helping you today. Most questions will involve troubleshooting, and usually take many posts back and forth.
I don't know where you got your part number, but I looked it up and I get 1600-819516A2. That is the Mercury marine part number for it. I'm not sure if you know, but Mercury bought Force a while back, used some Force designs along with Mercury/Force combination designs before phasing them out completely.
Now when you take my part number, 1600-819516A2, 1600 designates that this is an obsolete force part number, and your left with 819516A2. 819516A2 is the part number I get, so you might want to double check the number you have (as it does not come up a valid part number).
819516A2 is the only gearcase you can use on this engine.
Your options are
1. checking out that used one
2. Getting a new, but old stock unit at twice the price. Seamarine has at least 1 in stock right now.
3. Having your lower unit inspected by a prop shop that also handles gearcase welding. They will be able to determine if the crack is in a critical location or not, and if it can be welded.
4. Simply sell what you have for parts, and purchase a used outboard. I don't know where you are from, but in my neck of the woods (i'm in florida, boating capital USA) a late 90's to early 2000's 40 hp 2 stroke in good running condition can be had for under 1000. You can definitey pick up a complete parts motor for less than what the this scrapyard is trying to sell that lower unit for.
Other than that you can burn hours or days calling scrap yards all over the place and roll the dice with a used part.
Now the gearcase used on your outboard was only used from 92 to 94. So you have to find a Force 40 from those 3 years only if searching by year. Searching by serial number,0E000001 THRU 0E093699 is the range that they used that gearcase on. Or use the part number I gave you earlier.
Personally, 750 for a used lower on a small 40 hp is a bit on the high side. Especially if the condition is uknown. For that kind of money, and again this is personally. I would pick up a newer outboard and part out what I had to re-coup some of the expense.
Post back with questions