Great fun to drive sounds like you may have caned it, but i could be wrong of course...

Its juicier, lost power, doesnt rev up properly, lots of front end noise.

If it sounds a bit like a tractor, replace your hydraulic lifters in the head. If they failed, (over-revving is a killer for lifters and will prematurely degrade them at best) they wont retain engine hydraulic pressure any more causing them to open up a massive gap between the camshaft and the valve stem itself, creating lots of metallic impact noise. This fault will cause a lack of throttle response and lose power and economy due to a lack of incoming or exhausting fuel-air mixtures. Maybe even bad enough to limit the revs down to 3k or so by means of mechanical gas flow restriction (lack of valve lift). If its left and still thrashed, something inside wont like the continuous rapid impacting and will probably give up on you with very inconvienient timing.

In an internal combustion engine we have inlet valves, and exhaust valves, (both are inside the head) but not flap valves, which limit airflow to going in one direction. The air only ever passes through an engine in one direction, and so there is no need for a flap valve. It would be redundant. So i hazard a guess at saying there just isnt one attached to your car. Let alone saying its not active! On the flip side, if the inlet valve is the object in question, it still doesnt make sense because inlet valves are "one piece" and look like a 5inch rod or so, with a flat bottomed foot. No moving parts here, least of all, a flap attached to it.

Lifters could cost you £10/15 each or something, they vary.

Also check -

Spark plug electrode gaps. (should be 30thou +/-2 with feeler guages)

Throttle position sensor. (wires should read correct values at idle position and wide open, if not, relocate the sensor or replace it if the figures are not consistent)

Ignition timing. (Shine a timing strobe gun at the bottom pulley, connecting it in loop with plug no.1, and check that idle speed is operating with a correct amount of advance over the top dead centre mark, blipping the throttle should see the timing mark move towards the back, further advanced, while it accelerates and see it go the other way, retarded, when it decelerates, returning to normal at idle. If it doesnt this will cause major power losses and ignition control needs replacing)

Compression. (screw a guage into the spark plug hole and crank it fast without any other plugs fitted, open the throttle too to get maximum air into the piston for testing high pressure leaks)