For starters get a copy of "Tuning BL's A series Engine" and "How to modify your mini" both by David Vizard (aka God).
Ok, here is a quick run down on how to get more power:
All performance increasing methods can be classified into 2 distinct groups:
For a beginner look at the following areas:
Exhaust
Larger bore with less restrictive silencers lets the gas exit easily = more
power to wheels. Beware that a too large a bore will reduce gas speed. Fast
moving gas helps to suck the remnants out of the cylinders. 2" bore single box
(silencer) is a good one.
Carb
Is the carb mouth diameter big enough? Large/twin carb lets the engine
breath easier, but too large, again, reduces gas speed, necessary here to draw
fuel from the jet. Use HIF38 1.5" on 998, HIF44 1.75" on 1275+.
Inlet
Once the gas gets past the carb, can it flow smoothly to the head?
Avoid narrow/tight bends. Also keep the gas cool. Denser air holds more oxygen.
Plumb in water heated manifolds in winter only.
The three areas above are known as 'Stage 1' tuning.
If you're feeling adventurous look at...
Head
You can aid breathing by using larger valves and milling out the ports and
chambers. You can raise the compression ratio (CR) by skimming the head face
down to make the chamber volumes smaller. Stage 3 (big inlet, chamber/port work)
heads are good.
Cam
Longer cam durations make the valves stay open longer and fill/empty the
chamber better. Too long however will move the power band up the rev range and
make the idle lumpy. MG Metro, Piper 285 or Kent 276/286 are good.
Rotating parts
Crank/Clutch/Rods/etc will take less energy to accelerate if lighter.
Ignition
A more powefull spark can be got from a sports coil. More precise ignition can
be got from electronic systems. Use good plugs (eg: NGK). Don't bother with
multi-electrode plugs.
Gearbox
Straight cut teeth zap less power than helical cut teeth.
Differential
Using a lower geared diff (eg: 3.44:1 as opposed to 2.95:1) will< give better
acceleration but will mean higher revs when cruising.
The list is endless!