This is usually done by spinning the head (or head insert) in a lathe and turning out some material.
Now I’m not saying that you can’t do it with a Dremel, but it will certainly be tedious and difficult to get a consistent geometry
There is a lot of experimentation to be done with combustion shapes, volumes and squish band design. These designs are verified by dyno/track testing and then chambers are produced on a CNC machine to those designs for sale to give specific power characteristics.
In terms of just adding 1.5cc of volume, and keeping it as simple as possible, then it’s probably easiest to just follow the existing profile.
The procedure I would follow is to first weigh the head (or inset) on a set of precision 1kg digital scales (cheap off Ebay – sometimes known as drug dealers scales!!) and measure the volume with a burette. So lets say you do this on the engine with the piston grease sealed to the bore at TDC and measure a volume of 21.5cc including the plug hole. The target volume is to be 23cc. Now we want to remove 1.5cc of aluminium from the combustion chamber. The density of aluminium is 2.7 grams/cc, so we want to remove 1.5 x 2.7 = 4.05 grams of aluminium.
In the case of an insert, it might have a starting weight of 108 grams, so our target weight is 103.95 grams.
It’s much easier to just keep putting the head/insert on the scales rather than having to keep bringing the burette into play and installing/removing the head from the engine.
In terms of estimating how much to remove, it is really just guesswork until you gain experience, so at first just go at it slowly with frequent weighing.
Lots of different views on the best liquid to use in a burette, anything from very light machine oil or a 50/50 oil/paraffin mix to the degreasing fluid out of a workshop parts washer. Certainly not a heavy oil as half of it will be left sticking to the wall of the burette!
Yes the same would apply to a 125, but the target increase would be say 0.75cc. The increase in volume, just gives a free revving motor with better over-rev.