the 7.7mm Jap is the .303 British, with a rimless case. The bullets, powders, velocities and pressures are the same, or virtually identical. What you can do with one, you can do with the other. The only real difference is the case itself, and the rifles you find each in.
Since you aren't set up for rifle reloading, I won't bother to go over forming 7.7mm brass from .30-06 cases, but its can be done, and often was as the only way to get usable 7.7 Jap brass.
Today the situation is better, new brass can be found (but tis not cheap), I've gotten Graf brass at gun shows (though not lately, since I haven't been looking)
the "full boat" type 99 will have the receiver dust cover, the monopod, and the AA sights. And an intact "Mum" will increase the collector value.
Later production rifles did away with the AA sight (and I think at some point, the flimsy monopod) , and troops often did away with the dustcover, supposedly because it rattled and gave them away when on patrol. Personally, I think it was also because they could, without imparing the function of the rifle.
If you have an Arisaka in military trim, there is one "accessory" that is vital, and more so, in my opinion than the dust cover, and that's the bayonet.
If you don't have one, get one, the rifle just isn't "complete" without one.
