For unshrouding the spark plug I wrote an article for this before, just as a preamble, the idea of unshrouding the spark plug is so it is not hiding in a well. With easier access for the spark to be exposed to the fresh air-fuel mixture it will ignite a more robust flame kernel. The pathway from the ceiling of the transfers to the plug center is also partially obstructed by the "bumps" that raised the compression ratio. A slight shave to these bumps will clear that passageway. You will need to tinker about with your squish band after unshrouding because you would want to restore the compression ratio. The 5cc volume works well with the stock bore, you will need to make adjustments for the overbore.
Stock AV7-AV10 heads have two basic shapes: open chamber, which is hemispherical; and closed chamber which has a large protrusion on each side of the spark plugs.
The volume of these heads is approximately 5cc for the closed chamber and approximately 8cc for the open chamber.
Stock 5cc heads are slightly restrictive towards the flow of incoming charge from the transfers.
An easy fix is to remove some of the material shrouding the access to the spark plug.
Then restore the original volume of 5cc by shaving the head.
You would need a marker, some files, some alumimium oxide papers (220, 600 and 800 works well), kerosene and fine steel wool to finish to a good polished look.
Basically you need to shave down a strip from the bumps on either side of the combustion chamber.
The strip needs to be 3mm at the center of the strip and taper at both ends.
Then remove the extra material around the spark plug thread insert at the top of the chamber.
Using hand files and aluminium oxide papers this is a 20 minute job.
Use a permanent marker to draw on the head where you will be removing the material.
The material removed will be to leave a 45 degree ramp, which you would them blend into the surrounding material
so the roof of the combustion chamber is smooth.
Check that there is no lip at the front of the chamber, this lip should also be blended in so there is no pocket when the piston is at TDC.
Start with reworking the sides of the chamber.
After the sides are roughed out then move to the periphery of the spark plug insert.
Reworking this area calls for blending out the ridge between the chamber and the insert.
Carefully pare it down for a smooth transition.
After roughing out the material smooth out the transitions with 220 paper, then 600 and finally 800.
Use the paper wetted with kerosene. Finally buff out the work with fine steel wool soaked in kerosene and wipe out with a rag so she shines.
Next shave the head to restore the volume of material removed.
Use a syringe with some 2 stroke oil to measure the volume of the chamber.
Stock is approximately 5cc with the right sparkplug installed (Champion L86C or NGK B6HS or equivalent).
Note how much extra volume there is in the upper chamber.
If you are attempting to go for higher then stock compression ratio calculate the target volume you need before you start to mill.