No, the Athena is A LOT larger than stock. Also, it has actually not too much to do with displacement as the cylinder even in a 50cc configuration would put out many times the power and heat as stock. The compression ratio is much higher and it is not an issue of machining. Most of us did it by hand with dremels and racquetballs for Christ's sake.
The stock kit does not HAVE a squish. Not one that can be used for reference anyway. See that is where you are giving me the impression of doing it wrong. You think you can measure all the stock parts and that'll be "ok" and it won't. 6.5:1 compression will require so much head material removed or so much spacing that you will not actually have a squish band and will be losing a huge part of the reason there is performance in general. In that case get the cheapest kit you can, because it isn't going to make dick for power anyway.
Forget about trying to modify what is there and consider it to be making something that never was. You need to know what a squish band is, how it functions and how you would like to make yours. Every one is different and every one will have different results.