Exhaust/manifolds/carbie may be the quickest, easiest and probably the cheapest way (unless you port the head yourself) to get more power on a stock motor, but they won't give you anything near what modding the head will,
if the work's done properly. I think Vizard says as much as a 20% increase in power from good head modifications on a 12G202 - I might be a bit out, but it was a big number... Whatever the number was, he didn't have a high opinion of the gas flow of the majority of standard Mini heads. But there is scope for a good improvement. If you want more power, get the head worked first. The bigger carbs/exhaust etc will work better with a better flowing head. A comparison would be the roadworks you guys have down there. It doesn't matter if you've got 3 lanes each way for 400km, that 5km stretch with stop/go controllers and a 40km/h zone is what determines how fast the rest of the road flows. There's no point widening the 3 lanes out to 5 (bigger carbie and headers) unless you finish the roadworks and get rid of the 40 zone first. You can't fully utilize the extra capacity, due to that one congested spot that everyone has to travel through. The biggest restrictor on power (read: gas flow) in a Mini is the standard head. It's the 40 zone. That's why no one runs 750 Holley double pumpers and 3" headers on a Mini - they flow so much more than the Mini could ever use. That would be having 10 lanes each way going into a single lane 40km/h zone.
All that said, if you're going to work the head yourself, don't rush it. Have a good look at Vizard's book. He has photos and diagrams that clearly show what you're trying to achieve. In doing that, you want all four chambers in the head to be the same volume - known as having them cc'd. After doing all that, it would pay to get the head skimmed, to reduce the combustion chamber size after increasing the chamber size (which is inevitable if you're going to unshroud the valves, which is how you get more power). Increasing the chamber size reduces your compression ratio, which, though it does mean you can run really, really poor quality fuel, will dramatically reduce the power you can produce. And, if the head's going in to a machine shop to get skimmed, you should get hardened exhaust valve seats put in it, so you can run it on straight unleaded.
If that sounds scary to you, it might be worthwhile to pay someone else to do it.
Or buy one which has already been done (Sorry but I don't know where. Maybe someone does it on an exchange basis?).
I've read Vizard's book (a few years ago), and remember the theory, but I know it's way beyond my skills to execute it!
Sorry if I'm explaining basic gravity to a quantum physicist, but I get the impression that this is the basic sort of explanation that you're after.
With all that said, if you've got that spare head, you've got some reference material for where to grind away the metal (really, get (one of) Vizard's book(s), shouldn't cost you more than $70 - try eBay, or, for a new copy, here:
http://www.pitstop.net.au/view/marques- ... /plu/8858/ and
http://www.pitstop.net.au/view/marques- ... /plu/8313/ . It's worth the investment, it explains it all very well, others here will tell you which one they think is more appropriate), and you're feeling game, give it a go!
It'll give you the highest percentage power increase possible short of getting a turbo/blower or a bigger engine.
