It does indeed look to be a decent blade, though you are correct that the gonjo is missing. I wouldn't say that there is "nothing" you can do about that, but i'm not sure it would be worth you while to replace it since it would probably be far more than the cost of the keris to do so.
The blade and sheath appear to be Javanese. Even if the blade were not from Jawa i would say that it makes the most amount of sense to me to match the hilt to the rest of the dress rather than the blade. The outward appearance of your keris is usually best when it displays one unified cultural identity.
I am traveling at the moment, but i am sure that some of our members will be able to post examples of hilts that would suit this keris.