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Stiffer contra spring AND lighter rollers ?  You'll probably run the poor machine right into the revlimiter all the time 

Better try to change only one thing at a time, CVTs are a bitch to get right otherwise...

And take small steps only, from 6.5g to 5.3g sounds like a lot to me. My experience is that a difference of 0.25g is already noticeable in most cases (unless you're running into the revlimiter, but then it gets hard to tell any difference anyway...)

Well, if you already own a 180cc Ruckus, and just need a bit of an upgrade for the Agility as a reliable daily driver, I think it's fine to go with the 72cc indeed... With a decent kit, this will still be a very reliable setup, but it will be a lot easier to keep up with traffic, especially when you add a T2 camshaft for a bit of extra punch towards the top end...

I'm very pleased with my 85cc+T2 setup on the 12" A50 nowadays, but it's a bit early to tell whether it's as reliable as the 72cc setup on my 10" A50, which lasts for quite a while already and still shows no signs of wear...

My scoots both run the stock variator and belt + NCY and Malossi torquedrives + slightly modified stock airbox + stock exhaust + Bridgestone ML50 sticky tyres...  Roller weights on the 10"/72cc are chosen for maximum power, but for the 12"/85cc I went for a lower revving setup, since it has power to spare and I like the deep sound 

I've been disappointed by some variators other than stock, and I also don't like the idea of noisy dry-weather-only open air filters and flaky/noisy/way-too-shiny performance exhausts...

A 50,- 100cc BBK ?

100cc sounds like just a bit too much of an upgrade for a small 50cc engine...

I wouldn't be surprised if you ran into heat issues very soon, and crankshaft bearing issues some time thereafter...
And, at 50,- I wouldn't expect the kit to last very long either...

On the other hand, if you've got a scoot or engine to spare, and you're looking for a cheap kick.. 

I've opened the float bowl and the first thing that i noted that was obviously different was that the idle jet on the new carb stands proud of its location tube.... whereas the idle jet on the old carb sits deep inside the tube so it shrouds the jet (maybe this is why I was having idling problems me thinks? - maybe with the jet being shrouded it was struggling to get a good and regular supply of gas?)

Nope, it's just a different design... (unless it isn't and they just didn't turn it into place all the way, of course  )

With the actual passage being only .35mm diameter or so, it definitely won't struggle for gas... The .35mm opening is the limiting factor by design... Any shrouding around it will relatively be such a small hindrance to the flow that it's completely negligible...

His Agility starts first time every time, so I guess there's nothing wrong with the ignition itself. It might need a new plug though, it's possible that it has an insulation fault that doesn't show when cold, but my money is still on a carb/air leak issue...

@fabio : did you have a look at the plug right after the engine dies ? If you clean it before starting (preferably when the engine is warmed up, so the choke is off) and check after it died, it can tell whether you have to look again for an air leak or blocked passage in the idle circuit (clean plug), or whether it's running way too rich from e.g. a damaged idle mixture screw seat (black soot on the nose and/or top of the threads)...

Also, have you checked the float level and reliability of the float valve ?

Bypassing the regulator won't work since this is a shunt regulator, which is only active when the voltage gets too high... It doesn't do anything at idle anyway... The core of the problem is that the stator coils don't have enough windings to generate 12V AC at idle with a 45 Watt load or so...

There's nothing you can do about it, apart from making your own stator coils with a few extra windings, but then you would also have to find (or build) a regulator which can stand the extra power at higher rpms....

Oh, and if you're really adventurous and skilled in electronics you can rewire everything to a DC-only configuration, which also means you'll have to design a charging circuit which uses all the stator coils for charging the battery, instead of only the smaller coil..

I've been thinking about these solutions for a while myself, but solving this small problem is just not worth the trouble it will take... Trust me, I make a living designing and building custom electronics...

Best solution : adjust the idle rpm somewhat higher (but not so high that the clutch will engage of course), your lights will be brighter, and your engine will be happier since the oil pump will also give more flow of that precious juice which keeps it healthy  

1.5F is a _very_ large capacitor in electronics terms, the only way to make it a useable size at 12V is to use a technology that also makes it far less powerful than a good battery as far as I know..  But then again, I've been working in completely different fields in electronics for the last decade or so, maybe I missed some change in the laws of physics 

Anyway, on an Agility the lights are on the AC circuit.... A large capacitor acts as a short circuit for AC (simplified explanation, but still true enough in this situation), so it will stabilize things for sure : your lights would be in a very stable state indeed :  "OFF"