Geoffrey, as mentioned, there are numerous threads in our archives that cover the cleaning of keris. Here in one that is pretty good, but you may want to search our archives for "keris maintenance" or "keris cleaning" or "warangan" and read them all if you plan to embark on cleaning up this keris. If is were mine i would definitely clean it and attempt to restore the hilt somewhat, as it seems like it also needs a bit of attention and it is a somewhat rare style that should be maintained.
http://www.vikingsword.com/vb/showthread.php?t=23934
The first thing i would do is remove that hilt completely and try to rejuvenate it a bit with a good wood oil. As mentioned, keris hilts are pressure fit with cloth or string. You can remove that old tattered material and any that you find caught up in the pesi hole of the hilt and replace it with a new strip of cloth when the time comes to place the hilt back on.
I would reiterate Kai's advice to NOT boil the blade in saltwater. I don't think the salt is a very good idea. Some varnishes can be difficult to remove. Given the black colour this seems more like some sort of paint, but who knows what goes through the crazed minds of some of these museum folk when it comes to keris preservation. LOL!
If this was mine i would simply lose the current hilt cup and spacer, especially since it is not silver. I am not even going to continue calling it a "selut/mendhak" even in quotation marks because it is neither of those. These type of fittings do not seem culturally appropriate to this keris. It is not too difficult to obtain a decent mendhak for a reasonable price. They pop up fairly often on ebay or you can google "mendak" and find many for sale on the internet. I'm even willing to bet that if you put up an in-search-of on our Swap page that someone would come to your aid.
What you are looking for to match to this hilt is something similar to the examples i have attached here. There are lots of variations and some styles will be more appropriate than others dependant upon the origin of this keris. Alan has suggested that your keris may have a East Jawa origin. There are a few of that variety in the last grouping which one of Alan's photographs, so perhaps he could guide you to a correct style for this ensemble.