Re: 1963 Mercury 850
The carbs make the most difference.
Another diff was an aluminum exhaust baffle plate used in later models vs. the flat stainless plate used in the older 850-6. This revised baffle provided a bit of exhaust tuning and if I recall was the specified replacement for a bad original stainless plate.
One other thing, albeit not absolutely confirmed, is the reed stops. It's been posted elsewhere that the stops used on the 1000-6 are larger than an 850's. Merc did a similar thing on various other motors, but I only have reed stop info from '65-on in my manual.
For example, the '65 900-6 and '66-'67 950-6 used .1562" reed stops whereas all 100 hp models from '65-'69 used .1875". As did all 650-4 models and 1100-6's.
I'm assuming earlier 1000-6 models used the .1875" reed stop but perhaps one of the Merc Super Guru's has access to older info or has reed blocks from an older 850 and 1000 kicking around to measure the difference. The stops for an 850 do have a different part # than stops for a 1000, so that would sure lead you to believe such.
I'd expect that bolting a set of 100 hp carbs onto your 850 will make it a strong runner. I did a similar mod to an old 85 I had and it ran very well. Had no idea about the reed stops, either. It had an alum replacement baffle in it as well.
The larger reed stops would let the reeds open more, to allow max airflow/hp at the upper end, but I bet the carbs are over 90 percent of the hp difference.
If you could snag a good set of reed blocks from a later-model 650 or 1000, the blocks and reeds have a better design which likely flows more air, in addition to the larger reed stop.
Of course you'd need to pull the powerhead and remove the manifold/crankcase cover to install different reed blocks or change out reeds/stops.
I'd be inclined to try just the bigger carbs and the worst that can happen is you'd have to reduce the main jets one size or so. Or, since I expect your gas has ethanol in it, it'll run leaner anyway and balance itself out pretty well.
Definitely gonna have more top end than the small-throat 85hp carbs. And use more fuel too!
Cheers..........ed