That's a good start. The hardware's in decent shape so I think O-rings'll do it.
As far as actually working on it, after reading years of TWB horror stories of newbies taking markers apart I was really surprised how simple and crude a thing it is. Much easier to work on than the air guns or machines I'm used to. Apparently that didn't stop the previous owner from getting a ton of crusty pipe sealant into the valve body, though.
I'm going to stick with CO2 for now. HPA is too much of an investment--I'm not actually planning on playing the game--this is just another project to ad to the list for now. But if anyone invites me to go play some time I'll be equipped for it. Might got the HPA route if I get into PCP air rifles.