Does it have a closed cooling system?

Yes they do

What base motor did Mercury use when they designed it? Is it aluminum?

Mercury Racing QC4V

9.0L DOHC Big-Block From Mercury Racing (motortrend.com)

Its bespoke. I’d guess they kept the BBC bore space of 4.84” and maybe the 9.8” deck height. Cranks and rods might interchange, that’s it.

You are definitely correct in that it is bespoke.

The entire engine is metric;

Bore-116mm
Stroke-107mm
Bore spacing-125mm

Notice the odd bore spacing. A Chevy crank does not drop in. It’s also got an 8-bolt flywheel pattern.

Closed valley, non-Chev deck height (no lifters to account for). Diesel-style thermostat in valley controls closed cooling temp)

168T ring gear, Chev-style marine starter and alternator, rod and main bearings are all that is used from The General.

https://www.mercuryracing.com/engines/automotive.html

So cool....

How does this chain work the overhead cam? More chains or a belt?

What is Bespoke? Rolls Royce motor? Bentley? Just a term?

The word bespoke (/bəˈspoʊk/) has evolved from a verb meaning 'to speak for something', to its contemporary usage as an adjective. Originally, the adjective bespoke described tailor-made suits and shoes. Later, it described anything commissioned to a particular specification (altered or tailored to the customs, tastes, or usage of an individual purchaser). In contemporary usage, bespokehas become a general marketing and branding concept implying exclusivity and limited runs.

Who manufactures the long block for Mercury?

I know that Mercury sends the blocks out to a third party for boring and nickel plating the cylinder walls for rebuilds…