Where does the AM2 engine get its extra HP from when compared to the 635?

Is it just the compression ratio and forced induction?

Yes.

... and most of that increase is down to the higher CR, in my experience.

Thanks guys

Does the peak torque move with the increase in cr, or, is it added across the range?

My butt says peak torque move ..

Indeed the extra BHP is noticeable but only worth doing on a engine overhaul ,1 notable boost is to upjet the solex twin choke to Dyane jets ,

So, all stuff external to the engine block, as I thought.

Dyane jets?
Every Solex carburettor off an A series engine which I've got my hands on over the last 38 years has had the same jetting, regardless of whether it was originally installed in a car with normal or forced induction.

The earlier (21-24) versions had a 120 primary and a 70 secondary, the later (18-26) ones had a 102.5 primary and an 87.5 or 90 secondary, since Citroen either did not understand the effect of that 'mild supercharging' or couldn't be bothered to address it.

Since carburettors are primarily sensitive to intake gas velocity, with no capabilty to cope with changes in ambient air pressure, the mild 'supercharging' resulting from the Ami/Dyane intake off the fan cowling was enough to depress the level of fuel in the emulsion tubes, weakening the mixture delivered to the engine.

That was very noticable on the Dyane we bought new in 1980, with a flat spot bordering on a lean misfire on the first choke, a fault which Bristol Street Motors in Leeds failed to cure despite trying 3 or 4 replacement carburettors.
Once the car was out of its 12 month warranty, I drilled the 102.5 primary jet out to a 110 and the problem was resolved...

hey Ken I beg to differ , had a lot of 2cv-dyane engines pass through my hands back in the late 70's, I found the Mid 70's Dyane engines had larger jets , including the pump jet

Roco,
apologies, but what relevance has the accelerator pump jet size?
That's a transitional stage, providing a mixture enrichment, the duration of which is less than 5 seconds and only occurs after the right boot has been planted on the pedal...
.
Also, 2CVs didn't have twin-choke carburettors as a standard fitting until July 1978, which most folk would reckon to be 'late 1970s'.