I have got a Magnex stainless 4 branch & full two system on my 'H' plate MK4 EFi cab. It does sound lovely & still sets off the odd car alarm when i'm over 4000rpm.
It is quite an easy job to change the idle speed control valve.
disconnect 1 pipe & disconnect 1 multiplug then undo 4 torx screws. Make sure the paper gasket is not split or torn. The total time should be about 15 mins & that is with a tea break/ fag break.
It will have to be a strip down & rebuild to replace the bearings on either the diff or input shaft. Whereabouts are you?
If your near me I could come & have a look.
The only CVH Engine that got put in the MK5's was a 1.4 then the 1.6 engine was a baby zetec
The red thing to the right is an alarm, possibly a scorpion alarm unit
There is a allen nut at the front of the gearbox on the main casing & next to the fifth gear housing. You will need a 6mm or 7mm allen key & as eegsy said