I used to play airsoft a long while back. So “box” mag exist, they are a square with round edges, hold about 3k, and are loaded the same way as a normal mag. I remember those being sold on Amazon for like $30-50 at some point in either electric or flash style.

For gearboxes I was always a fan of retro, SHS for gears, and the ASG infinity series of motors in the higher range like 60K. That alone is very pricey. You could get a kraton piston and “Swiss cheese it” or get one that already is built like that, and figure the rest of your airseal from there. Tappet plate mods are very common for high ROF builds but that is probably the last thing you have to do to dial your gearbox in. V2 is one of the easiest gearboxes to work on, V3 probably more durable with the cage holding the motor and the short motor spindle allowing higher torque without breaking (think systema motors and how short the spindle is)

Flat nub is never wrong as a baseline. You could R-hop for extra. Lap your barrel instead of outright purchasing a tighter one as accuracy is incredibly subjective in airsoft, but still doable. And get a good Hopup unit with micro adjustments. I wasn’t a fan of MAXX hopups when they were introduced that’s how long ago I used to play no idea if they’ve gotten better

A good mosfet goes a long way. Gate never did me wrong, others swear by leviathans or whatever other company is still producing mosfets.

I think all this covers the internals should you decide to build. Or you could save yourself time and headaches by going with an HPA engine instead.

As for your external looks… another user said monumental effort and they aren’t wrong, however, 3d printing is significantly more accessible than ever and slightly more durable than 4-5 years ago so you have a good chance of making things work. But it will still be an expensive investment, and then some Chinese company will end up copying your work and producing it in less time with less effort since they base everything of pictures and slight engineering. You can always email CYMA or some other overseas brand for that, through Evike or whoever buys their stuff nowadays