The inner barrel is 285mm, a 300mm fits.
Don't try to unscrew the flash hider! There's a set screw inside.
A 10.8v stick or 11.1v lipo stick will fit, but I would upgrade the motor & compression parts first.
The rear sling mount is on the "wrong" side & sounds like a cowbell when it hits the stock. But, I wrapped my stock w/ paracord & added a slip-on pad that makes it comfortable to shoulder.
CYMA's QD AK scope mount works, but you must flip the rails around or you will not be able to use it on a 74U. When you attach the scope mount, you can't lock your folding stock on the receiver.
The pistol grip on my gun is not rubberized, but a very sturdy plastic. If you're planning to replace this grip w/ the ergo one seen on other CYMA AKs, I tried, the motor grip screw hole doesn't line up. Mod it or...wrap the grip like the end of a hockey stick.
The motor is weak for my taste, but there are plenty of motors to choose from here on Air Rattle.