As far as i know the bolt on modifications you can do on the 1 litre engine are: stage 1 exhaust, LCB and K&N filter (20% power increase), and changing the cylinder head with a ported and better flowed 12g295 from a 998 cooper s (+6 hp). This without spending too much money. If you can and want to spend more then you can:
put a lightened timing gear, flywheel and clutch
change the camshaft, megadyne 256 or 266, these woul give you better torque at low and mid range rpm's respectively, don't go for anything higher than 270, you won't get any benefit unless you are always pushing the engine higher than 6000 rpm.
As far as i read there is no advantage really in changing the rockers with 1.3:1 high lift, again unless you re pushing the engine all the time over 6000 rpm.
You can install a ram pipe to increase air flow in the carb, but the gains are a max of 4-6 hp, according to the golden pages Vizard manual.
Then you can go for +40 or +60 omega pistons, but at this stage you are redoing the whole engine.
Or you can simply put in a 1275 engine.
Now let's see how much the experts will kick my ass
