Okay, wow, a bit to break down here.
Adding a save feature alone would be a hell of a feat for a rom hack. And since it a rom anyway, why would you even build in a save feature? You would use the emulators state save.
Please read all the text before responding:
1a) Some sort of save feature would be good, if we're talking something not running on an emulator.
I made that 1a as more of an aside to the key issue, and per the bolded text, I'm talking outside of an emulator, like some sort of official remake or re-release.
Weapon storage and selection require binding the key controls for it, and increasing the game memory to create an actual inventory.
I'm not sure where talk of binding and inventory is coming from.
In the game, you're allowed one subweapon per Turtle. As you're walking along, bashing enemies' heads in, they can drop another item, and there's just a slight delay to it, making it far too easy to pick up without realizing it. Suddenly, your 99 scrolls just became 20 shuriken.
Make that 19.
Because whenever you pick up a new subweapon, it's switched on automatically, whether yours was or not. So now you're trying to fend off enemies with your normal attack and you're instead wasting ammo of the new weapon you just got without realizing and didn't even want.
Or just do away with random weapon drops altogether. I won't miss 'em.
Some of the stuff you want could probably done with minimal problems in the game code. Some of the stuff you want requires a fundamental altering of the game engine. Since you're asking for that kind of stuff anyway: might as well just start from scratch in an engine that lets you do that stuff to begin with instead of trying to hack a 32 year old, 8 bit game.
Plus, you'd be able to get better graphics, tighter controls, and a whole host of in game feature adjustments for your trouble.
More work? Yeah, absolutely. But the end product would be worth it.
A guy in his garage could probably implement the changes I'm talking about. Hell, I could probably do it, if I had enough time to dedicate to learning how, but I don't.
But rebuilding it from the ground up is going to take way more than I've got the time or resources for.