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It’s probably between the years ’69-72. After that the low-tension magneto was used on these. But there are people who revert the 73-76 models back to the universal magneto because they are generally easier to deal with. Your cylinder head doesn’t have the bosses for the external coils, so unless someone took the trouble to switch cylinder heads, you can probably bet it’s original to the block. Do the colors match? Did the head gasket seem ‘married’ to both the cylinders and head? If yes, that is just more evidence that it’s original to the block. I doubt anyone who go to the trouble of switching out the crank and manifold, along with the carb. Unless you have someone spending a winter on it as a hobby (long shot).
In terms of the impeller housing screws, I can’t comment on which ones you should use without seeing the housing, but of course you won’t be able to use the smaller diameter ones with a housing calling for the larger ones, without using some very small SS washers to secure the housing to the gearcase. If the threads are boogered up, well it’s not hard to retap the gearcase orifices being that it’s aluminum. You can get away with 2 out of the 3 screws so long as they are opposite of each other. If they are next to each other I would consider that iffy in terms of exhaust getting in.
And make sure that little relief hole in where the water tube goes into the housing is clear (you have to remove the rubber water tube grommet to see through). Those 2 O-rings on the spacer need to be new, soft, and snug, otherwise it will cause water flow to be affected by the exhaust pulses of the motors.
When you move up to the ’77-thru motors, if those O-rings are PERFECT, it will literally shut the water off completely like a faucet! Those 32 C.I. motors breath a lot more than the original 22 C.I. ones!
Check your compression numbers. They’ll run at anything above 90, but 120+ is more desirable. A really good condition unit will show 150PSI. I have seen extraordinarily low-hour/well-maintained units hit 165PSI on my gauge for the 25hp variety. The 18/20’s, best numbers I’ve seen is 135PSI. Note these are cold/wet measurements. After running, the numbers will change.